Take your TB off and clean it if you haven't already. Carbon build-up likes to hide behind the butterfly and make it stick and cause issues. Go to the JY and grab the ICM from another Grand Prix or Regal or anything with a 3800 really. The ICM is the silver piece that the coils connect to. Do those things and see if your issue goes away. Have you had the car scanned since buying it? Are there any dash lights on? I would have the car scanned as well for good measure.