I have a maruti800 1998 model and it has run 20000 km till now. Now I have a problem with my vehicle , at lower speed it starts jerking and at higher speed it runs smooth. Petrol filter and air filter are ok.In first gear also during starting it jerks but as I accelerate it then it runs smooth.

This vehicle had option of three different transfer case ! Which one of these is the one in your vehicle ?
Transfer Case - NVG 120-NR9 Transfer Case - NVG 126-NP4 Transfer Case - NVG 226-NP8
The New Venture Gear model NVG 226 RPO NP8 transfer case is a two speed automatic, active, transfer case. The NVG 226 provides five modes, Auto 4WD, 4 HI, 4 LO, 2 HI and Neutral. The Auto 4WD position allows the capability of an active transfer case, which provides the benefits of on-demand torque biasing wet clutch and easy vehicle tuning through software calibrations. The software calibrations allow more features such as flexible adapt ready position and clutch preload torque levels. The technology allows for vehicle speed dependent clutch torque levels to enhance the performance of the system. For example, the system is calibrated to provide 0-5 lb ft of clutch torque during low speed, low engine torque operation, and predetermined higher torque for 32 km/h (20 mph) and greater. This prevents crow-hop and binding at low speeds and provides higher torque biases at higher vehicle speeds, to enhance stability.
The New Venture Gear model NVG 126 RPO NP4 transfer case is a one speed automatic, active, transfer case. The NVG 126 provides only one mode, Auto 4WD, and is transparent to the operator. The active transfer case provides the benefits of on-demand torque biasing, wet clutch and easy vehicle tuning through software calibrations. The software calibrations allow more features, such as flexible adapt ready position and clutch pre-load torque levels. The technology allows for vehicle speed dependent clutch torque levels to enhance the performance of the system. For example, the system is calibrated to provide 0-7 N?·m (0-62 lb in) of clutch torque during low speed, low engine torque operation, and predetermined higher torque for 32 km/h (20 mph) and greater. This prevents crow-hop and binding at low speeds, and provides higher torque biases at higher vehicle speeds, to enhance stability
The New Venture Gear (NVG) model NV120, RPO NR9 transfer case is a single speed, full-time, all wheel drive (AWD), transfer case that incorporates a front/rear drive differential carrier assembly.

Unplug each wheel sensor individually to pinpoint , bad speed sensor possibly

It is higher kms than average, which is reckoned to be max 20000 Km/yr for some reason. Drive.com.au shows your car to be worth approx $2400-$3700. A car at the higher end of this would be in good condition, 240000 km or less. Your car would probably attract offers at the low end. Sorry for the dampening news.

indicates water in harness connections or at the terminals of the ECM or other modules

An EGR valve can cause these problems, i had same exact problem, so replaced the egr and problems were gone.

Sounds a lot like the fuel pump for a starter. When it acts up does the vehicle jerk or buck before picking up speed? I have also had the experience with just one spark plug wire or plug bad. You can check this by pulling each wire one at a time while engine is running. If the motor slows that plug and wire are fine. If no change in rpm check the plug and wire.

takes a long time to run the rediness test since there are 6 different tests to complete...

PROCEDURE 1
EVAPORATIVE EMISSION SYSTEM LEAK MONITOR
Inspection Conditions
Engine coolant temperature: 30°C (75°F) or less (The engine is stopped before the test drive is started).
Atmospheric temperature: 5 - 30°C (41 - 113°F).
Condition of A/T: Selector lever D range.
Fuel remained in fuel tank: 30 - 50% is recommended.
Time required: 16 minutes or less after started the engine.
Drive cycle pattern: One trip monitor (from start to ignition switch to "LOCK" (OFF) position. Well be completed while traveling with the following drive cycle pattern.
Test Procedure
Engine: start.
Accelerate until the vehicle speed is 89 km/h (55 mph) or more.
While keeping the accelerator pedal opening degree constant, keep the vehicle speed at 89 km/h (55 mph) or more and travel for 16 minutes.
Return the vehicle to the shop, then turn the ignition switch to "LOCK" (OFF) position.

PROCEDURE 2
FUEL TRIM MONITOR
Drive Cycle Pattern
Inspection Conditions
Engine coolant temperature: 100°C (212°F) or less.
Atmospheric temperature: -10 - 60°C (14 - 140°F).
Condition of A/T: Selector lever D range.
Time required: 30 minutes or more.
Drive cycle pattern: One trip monitor (from start to ignition switch to "LOCK" (OFF) position). Will be completed while traveling with the following drive cycle pattern.
Test Procedure
Engine: start.
Accelerate until the vehicle speed is 89 km/h (55 mph) or more.
While keeping the accelerator pedal opening degree constant, keep the vehicle speed at 89 km/h (55 mph) or more and travel for 30 minutes.
Return the vehicle to the shop, then turn the ignition switch to "LOCK" (OFF) position

PROCEDURE 3
CATALYTIC CONVERTER MONITOR
Drive Cycle Pattern
Inspection Conditions
Atmospheric temperature: -10°C (14°F) or more.
Condition of A/T: Selector lever D range.
Engine coolant temperature: Not set.
Time required: 16 minutes or more.
Drive cycle pattern: One trip monitor (from start to ignition switch to "LOCK" (OFF) position). Will be completed while traveling with the following drive cycle pattern.
Test Procedure
Engine: start.
Accelerate until the vehicle speed is 89 km/h (55 mph) or more.
Travel for 300 seconds or more while keeping the vehicle speed at 89 km/h (55 mph) or more.
Decelerate until the vehicle speed is within 80 km/h (50 mph) or less.
While traveling at 56 - 80 km/h (35 - 50 mph) for 10 minutes or more, fully close the throttle at least once in 2 minutes and decelerate for 10 seconds or more.
Do not repeat deceleration too often.
Vehicle speed may go below 56 km/h (35 mph) after the deceleration.
Stopping and braking are permitted. (If stopped or drive at 56 km/h (35 mph) or less for more than 5 minutes the monitoring may be stopped. In this case please restart monitoring from the beginning.)
After completing the above deceleration, bring the vehicle speed back to 56 - 80 km/h (35 - 50 mph) and keep it in the range until starting the deceleration again.
Repeat the above deceleration at least 5 times.
Return the vehicle to the shop, then turn the ignition switch to "LOCK" (OFF) position

PROCEDURE 4
HEATED OXYGEN SENSOR MONITOR
Drive Cycle Pattern
Inspection Conditions
Engine coolant temperature: 100°C (212°F) or more.
Atmospheric temperature: -10°C - 60°C (14 - 140°F) or more.
Condition of A/T: Selector lever D range.
Time required: 16 minutes or more.
Drive cycle pattern: One trip monitor (from start to ignition switch to "LOCK" (OFF) position). Will be completed while traveling with the following drive cycle pattern.
Test Procedure
Engine: start.
Accelerate until the vehicle speed is 56 - 80 km/h (35 - 50 mph).
While keeping the accelerator pedal opening degree constant, keep the vehicle speed at 56 - 80 km/h (35 - 50 mph) and travel for 16 minutes or more.
Stopping and braking during this operation are permitted. Keep the accelerator opening degree constant for 1 minute or more after each acceleration.
Return the vehicle to the shop, then turn the ignition switch to "LOCK" (OFF) position

PROCEDURE 5
EXHAUST GAS RECIRCULATION (EGR) SYSTEM MONITOR
Drive Cycle Pattern
Inspection Conditions
Engine coolant temperature: 100°C (212°F) or less.
Atmospheric temperature: 5 - 60°C (41 - 140°F).
A/C switch: OFF.
Condition of A/T: Selector lever D range.
Time required: 16 minutes or more.
Drive cycle pattern: One trip monitor (from start to ignition switch to "LOCK" (OFF) position). Will be completed while traveling with the following drive cycle pattern.
Test Procedure
Engine: start.
Accelerate until the vehicle speed is 56 - 80 km/h (35 - 50 mph).
While traveling 16 minutes or more at 56 - 80 km/h (35 - 50 mph) with engine speed 2,000 r/min or above, fully close the throttle at decelerate for 5 seconds or more until the engine speed reaches 1,000 r/min or under.
Do not repeat deceleration too often.
Stopping and braking are permitted (Rapid deceleration and sharp steering are not permitted).
After completing the deceleration, bring the vehicle speed back to 56 - 80 km/h (35 - 50 mph) and keep it in the range until starting the deceleration again.
Repeat the above deceleration at least 8 times by fully closing the throttle valve.
Return the vehicle to the shop, then turn the ignition switch to "LOCK" (OFF) position

PROCEDURE 6
OTHER MONITOR (Main Components, Sensors & Switches, Wire Breakage & Short Circuit)
Drive Cycle Pattern
Inspection Conditions
Engine coolant temperature:
100°C (212°F) or less (Except engine coolant temperature sensor monitoring).
30°C (86°F) or less (Engine coolant temperature sensor monitoring).
Atmospheric temperature:
5°C (41°F) or more (Except engine coolant temperature sensor monitoring).
-10°C (14°F) or less (Engine coolant temperature sensor monitoring).
Condition of A/T: Selector lever D range.
Time required: 21 minutes or more.
Drive cycle pattern: One trip monitor (from start to ignition switch to "LOCK" (OFF) position). Will be completed while traveling with the following drive cycle pattern.
Test Procedure
Engine: start
Accelerate until the vehicle speed is 56 km/h (35 mph) or more.
While keeping the accelerator pedal opening degree constant, keep the vehicle speed at 56 km/h (35 mph) or more and travel for 16 minutes or more.
Return the vehicle to the shop.
After stopping the vehicle, continue idling for 5 minutes, and then turn the ignition switch to the "LOCK" (OFF) position. Moreover, the vehicle should be set to the following conditions for idling.
A/C switch: OFF
Lights and all accessories: OFF
Transmission: Neutral

it sounds like air in the fuel line or a carburator/fuel injector problem

is the fuel old?

my old vehicles that have been sitting run MUCH better if I give them new fuel

typically, this sounds like a wheel bearing. To test, the noise can often be altered by accellerating /decelerating, and more evident by turning the vehicle.

The truning process is to put load on one side, then the other. if the right side has a damaged bearing, for example, the noise would be noticibly altered (less in this case) when turning to the right. Load would be slightly less. Testing speed in the area of 25 to 35 mph.

doc

This is going to be either a programing issue in the transmission control module or an main control valve body concern. This vehicle is under warranty for 48 months or 50000 miles so I would have my local dealer take a look at it.
Ford has a new techincal service bulletin out on a concern similar to what you are describing. It states that if vehicle has less then 20000 miles to reprogram the PCM and TCM, if it has over 20000 miles they recommend to replace the main conrol valve body. The TSB number is 08-24-01