This is a common problem among Audis.

Symptomes

What happens is sometimes at start up the car idles very strange. The RPMs rise and fall from 250-1100 rpm before evening out. This "bounce" does not ussually last long; however, if you watch the battery gauge you will see that it fluxuates in proportion to the "bounce". This problem persists every time the RPMs drop below 1000. So when stopping at a stoplight this "bounce" occurs again. Sometimes this "bounce" will be so severe as to shut the car off and leave you without power.

Description

The cause of this lies within the computer. If the electrical system is altered this ussually occurs. If something passes thru the computer box to reach the firewall this ussually happens. I have never had/heard of this happening out of the blue; there is always some kind of interfearence. Some times opening the hood will cause this "bounce"

Solution

If you have routed anything through the firewall via the computer box, take it out or insulate it and make sure it does not come in physical contact with computer. This serves as a ground for the computer and causes Small short circuits.

If this started happening out of the blue, wait it out. Drive your car as much as you can. You will notice that after a while of driving (15-45 minutes) the "bounce" seems to diminish or maybe even be non-existant. If possible bring an extra car battery and jumper cables in case your car dies Eventually this problem will subside. Ussually in about a day or so.
The only time this did not diminish for me was when i had a nitrous line that ran through the computer box and was shorting out the computer. once removed the problem went away after driving and time.

ANother possible solution is to put your key in the ignition and turn it to the start position. Do not start the car, instead allow the car to sit for 3 or 6 minuets. you will hear a "whine" coming from under the engine. This is from the TB recalibrating.

Doose and I have had this problem and these were the things we did to get rid of it. Right now my car adles from 900-1200 rpm but that is b/c of the cams