[Andyuk911]

This Audi dealer you are using sound useless, which does not help much. They should be able to run Vagcom and see if the car is clocking up errors.

This could be core plugs, mega bad fuel ...

You have no engine check light ??

Suggest you take to another dealer and speak to senior tech ..

You could ask on RS6 , RS246 or Audisrs ....