Audi C4 AAT: Idle unstable, stalling sometimes

Hello everyone,
I recently bought an Audi A6 2.5 TDI, model year 1995, with the AAT engine.
"For about two weeks now, the idle speed has been acting up. It doesn't happen all the time, but when it does, it's quite noticeable. Initially, there are slight fluctuations, sometimes leading to the engine stalling. If I give it a strong burst of gas while idling, it usually works for a while, but the problem soon returns. Then there are times when everything seems fine. Any idea what might be causing this?"
Greetings, Rudi.

My first thought would be that there's air in the fuel.
Furthermore, there are crankshaft position sensors or needle lift sensors, but those should be stored in the error memory.