I just had the same problem on a 99 gtp i just bought. what you have change are the most common problems.
but the other common problems are the lower intake gasket, supercharger gasket, throttle boby gasket. you can check thoughs with a can for carb or brake cleaner. with the car running spray around the base of the gaskets to see if the races or stumbles. it could also be the throttle positioning sensor, if you replace that it does need to be recalibrated. I am will to bet it is one of the gaskets though, I ended up changing all 3 on mine. hope that help.
