car bogging down, not getting all power

so i bought a 2001 gti with all the upgrades already installed. just as i drove it home like a 30mile drive the engine light came on, so we brought it to the dealer and they didnt know what was wrong. so i was driving it and all of a sudden it started jolting and than it wouldnt alcellert good, if i was supposed to be in 2nd gear i would have to put it in 5 gear to move with out it jolting, and than when it jolts it dies at every stop sign. so we put about $3000 trying to fix it but it never was fixed the place we brought it to has no idea what wrong, they put a new chip in it and programed it. if anyone has any idea whats going on please help out

cant really help you w.o knowing the aftermarket parts.. 1.8t or vr6

"with all the upgrades?"

... whats that mean. mine didn't come with any "upgrades"

It Is More Than Likely Your Mass Air Intake Sensor Because My 2001 Vw Did The Same Thing In The Garage And It Wouldn't Go Over 1800 Rpms. A Few Hundred Bucks Will Fix That. This Problem Usually Happens When The Car Isn't Driven In A While.