my audi is idling low and chugging while idling but runs good at higher speed. what could be the cause?

Idle air control valve is a suspect. But should be throwing a diagnostic code.

And if it's not throwing code? I think it was reading oil pressure low. He's going to try and clean the screen in bottom of oil pan. Could that be it. Acts like a car that had had dirty gas. What about a fuel pressure rail regulator?

Could be something as simple as a vacuum leak. Particularly any of the lines under any center intake manifold cover and behind the motor; they get really heated, brittle and then crack. Sometimes they will seal when they are heated but leak when they are cold, so it makes it seem like the engine runs bad when it is cold. Simple stuff first. Break out the dish soap and check your vacuum lines. I have found my wife's bubble bath works great too. She gripes though when i get greasy fingerprints on the bottle though. Good luck.

It has been checked for leaks.