Car loses power but doesnt shut off. Reduced engine performance comes on. Changed turbo control valve . Problem shows up less often. What could it be

It's going to be hard to diagnose without looking at it. If the dealer and shop say it needs those two items they do sound inline with your issue. I suggest getting a second opinion. Use this link to find a good shop near you.https://repairpal.com/repair-shops Alex

Common issue with this car is the ETM electronic throttle module. The sensor in it becomes dirty and the car goes into limp mode. I would clean it and put some dielectric grease on the plug. This may be your problem.