Speed is lesser and smoke releases through salencer

Cruise Control
Any given speed above approximately 22 mph (35 km/h) can be maintained with the cruise control by operating the lever.
-see photo

1. Accelerate and set: Lift lever briefly to set speed. Hold lever up to accelerate.
2. Decelerate and set:
   Depress lever briefly to set speed. Hold lever down to decelerate.
   Normally the vehicle is accelerated to the desired speed with the accelerator. Speed is set by briefly pushing
   the lever to position 1 or 2. The accelerator can be released. The speed can be increased (e.g. for passing)
   by using the accelerator. As soon as the accelerator is released, the previously set speed will be resumed automatically. If a set speed is to be increased or decreased slightly, e.g. to adapt to the traffic flow, hold lever in position 1 or 2 until the desired speed is reached, or briefly tip the control lever in the appropriate direction for increases or decreases in 0.6 mph (1 km/h) increments. When the lever is released, the newly set speed remains.
3. Canceling

To cancel the cruise control, briefly push lever to position 3.
When you step on the brake or the vehicle speed drops below approx. 22 rnph (35 km/h), for example when driving upgrade, the cruise control will be canceled.
4. Resume
If the lever is briefly pushed to position 4 when driving at a speed exceeding approx. 22 mph (35 km/h), the vehicle resumes the speed which was set prior to the cancellation of the cruise control. The last memorized speed is canceled when the key in the steering lock is turned to position 1 or 0.
Note:
If the engine does not brake the vehicle sufficiently while driving on a downgrade, the speed you set on the cruise control may be exceeded and you may have to step on the brake pedal to slow down.
As soon as the grade eases, the cruise controlled speed will again be maintained as long as the brakes were not previously applied, or the lever may be used to resume the previously set speed if the brakes were applied.


http://www.benzworld.org/forums/attachments/w202-c36-c43/153145d1197313761-pdf-owners-manual-w202-1995-2000-1999-c280_owners_manual.pdf

Automatic vehicle at the beginning will automatically forward, but the speed is not too fast, because the automatic mechanism principle, high running loose throttle speed decreased than manual slow some, is a normal phenomenon. use gna 600 to diagnostic If the initial automatic acceleration, speed has a problem, the problem generally appear in the idle adjustment problems, such as: solar term door fault, the intake system leakage phenomenon, cause idle setting, you can go to your car repair station for inspection repair, in order to accurately determine fault.

Don't know about the speed sensor, but your spark plugs and air filter could be a problem with the black smoke and rough running.

black smoke is flooding( uncontrolled injector operation
run the fault codes to determine the problem area ( ECM , fuel pressure , faulty injector control, MAF/VAF sensors or failed heated oxygen sensors

use a couple of cans of injector cleaner as faulty injectors are indicated

if there is also black smoke coming out the exhaust - and the problem is intermittent - then it's the EGR valve - it sticks open - will need replacing.... Happened to mine this morning and this is what the Jaguar Assistance guy said. Mine is a 2006 2 litre X-type.

Sounds like you need a new speed sensor in the transmission,both the speedometer and the transmission rely on a signal from it to work properly.

If set at a specific speed and you have to slow down by hitting the brakes, The brakes turn off the cruise control. Pressing resume automatically turns the cruise control back on and accelerates to the previous speed setting. You can also use it to accelerate and by releasing it the new speed will be set.

hello,good day,any speed approx. 40 km/h can be stored.accelerate the car with accelerator or the switch to the desired speed and briefly press switch in position 1 or 2,,once the speed is stored,release the accelerator pedal,,,to increase or decrease speed in step of 1 km/h,briefly press switch in position 1 or 2,,stored speed can resumed from approx. 40 km/h,briefly press switch in position 4,(page 80),,ty

I have the same car. So far no problems with the Transmision. I had the Trans oil change out every 60 K Sure sounds like you need to get the Transmision checked. Is the fault light on. I have invested ina fault code reader sure is worth it. 
Good luck.  Vilmos