After a minute of driving my 1998 audi a4 the gear bar highlights and the car feels like its in 5th gear

Now automatic transmission is stucked in "limp home" mode. Even gears are not locked, transmission works in degradation mode. You might access the Transmission Control Module (TCM) using diagnostic equipment connected to the data link connector of vehicle, in order to pull out all Diagnostic Trouble Codes (DTC) memored. Then you will know if there are bad piece(s), and you will can replace bad piece(s) & repair damages. After that you will be able to erase all DTC from TCM memory, and reset TCM Electronic Control Unit.
Though you can check & change transmission fluid before, and test again the car.
For transmission oil filter changement you need to remove the gearbox oil pan.
Here you are all instructions about how to check & change oil in automatic transmission on 1998 Audi A4
At the bottom of the automatic gearbox (tiptronic) you will find 2 threaded plugs on below surface. First (in front of the car) is for emptying, and second is for filling up. Tightening torque is 24 Nm.


Automatic Transmission Fluid (ATF) checking:
1. Requirements for check:
1.1. Vehicle standing on level surface.
1.2. Transmission must not be in fail-safe mode.
1.3. Shift lever in "P" position, parking brake applied.
1.4. Engine idling.
1.5. Air Conditioner and heater OFF.
1.6. ATF temperature must not exceed 104 Fahrenheit degree (40 Celsius degree) at start of test.
1.7. Only ATF with designation ESSO LT 71 141 may be used. Do not use any lubricant additives.
1.8. Automatic Transmission Fluid ATF - part number: G 052 162 A2 (1 liter wrapping)
1.9. An ATF level check when transmission oil temperature is too low results in overfilling, when too high results in underfilling.
2. CAUTION: Too much or too little ATF will affect the operation of the transmission. The ATF level must be checked at regular intervals.
3. The oil level is correct if a slight amount of oil runs out when ATF temperature is between 86 Fahrenheit degree (30 Celsius degree) and 113 F deg (45 C deg) (caused by increase of oil level when heated). For warmer climates, 122 F deg (50 C deg).
4. Hang filled ATF reservoir VAG 1924 as high as possible.
5. Shift lever in "P" position, engine running at idle.
6. Place oil drip pan under transmission oil pan.
7. Remove ATF filler plug (at the bottom of the automatic gearbox you will find 2 threaded plugs on below surface. First (in front of the car) is for emptying, and second is for filling up.
8. With ATF temperature between 86 Fahrenheit degree (30 Celsius degree) and 113 F deg (45 C deg), for warmer climates, 122 F deg (50 C deg), a slight amount of oil will flow from filler hole when ATF level is correct.
9. If necessary drain or top off ATF to attain correct oil level. Top ATF with filler hook from VAG 1924 ATF reservoir. Insert VAG 1924 (tool) ATF reservoir filler hook into filler hole and top off until a slight amount of oil flows from filler hole.
NOTE: Fit filler hook into one of the oil deflector cap slots placed on the filler opening.
CAUTION: Do not press filler hook upward or oil deflector cap can be pushed off.
10. Reinstall ATF filler plug. Tightening torque is 24 Nm.