Ok where to go next.

Basics?
partially clogged injectors and/or fuel filter


What i would want to do is get the car on a scanner and see what values you have on the sensors for cold as opposed to warm. That may point in a specific direction. I would also want to see where the car starts to idle fine. Does it clear up once the ecu goes into closed loop or just a specific temp. Is the condition it clears consistant?

When cold the ECU works with a set enrichment. The ECU doesnt monitor mixture to conpensate. When warm the ECU reads o2 and adjusts the mixture to meet the targe AFR. That could be why you dont feel the problems at high rpms liek you normally would with clogged injectors. Then again it might be leanign out top end and you just dont realize it. Poss bad gas as well.

Terry