[Underwood]

I'm experiencing the same problem myself.

See my signature for my complete upgrade list, the problem first started to occur when the car got flashed.

Have you had the car checked out once more perhaps? Any answer to the problems?
My problem is even more present if running 3rd full throttle with three passengers in the car. Feels like the car has asthma sometimes.

I'm hoping it's not the clutch aswell....