My friend drives a 2002 1.8t jetta. He has some aftermarket parts on it, blow off valve, front mount inner cooler, and the exhaust was welded back on, along with the cats and has a small leak. He had his error codes cleared, and now the car jolts. Like I said, the car ran great before this. Error codes cleared, and it jolts like it's having a hard time shifting. Please help!