This sounds like a faulty knock sensor..
I had the same problem whit my Audi 90 20v..

Try to cleane the knocksensors connection remove corrosion and spray whit connectionspray.

If this did´nt help try to remove the knocksensor, cleane it upp and the surface, it has to be flush against the car.
Reassemble and re touqe whit about 10Nm

Some times these trix ill do the jobb, in my case the car was better for a while, but then i knew the problem and just relaced the knock sensor and after that the car turned upp fine.

Sorry for my bad English (I´m from Sweden)
I hope this will solve your problem...

Audi V8 3.6L -91 Audi 90 2.3L 20V -90 Audi 90 2.2L -86