I had this same symptom on a '91 100. It turned out to be the idle
actuator switch on the throttle body not making contact at idle. This
switch tells the ECU that it is either WOT or idle. When idle the ECU
and idle stabilization operate the idle stabilization valve (ISV) to
maintain idle speed. On the back of the throttle body there is a plug
with three wires, one is ground the other two mak contact when it is
either WOT or idle. Use an Ohm meter to check if it is making contact
when the linkage is at idle. If it isn't there is a small adjuster with
a lock nut that corrects it.

If it is making contact correctly you probably have a dirty sticking ISV
that can be removed and cleaned with brake fluid. You may want to check
the tech section for the T44s on Audi world to be sure what cleaner to
use. I think brake fluid is the choice by not sure.

In my case adjusting the switch on the TB was the fix.