i had this when i first got my gti, have you got vag-com? if so have a look at what numbers your hall sensor is showing.. when you start the car they normally change (hopefully into spec) mine werent changing no matter what so i put a new dizzy on and hey presto.. turned out the hal sensor was knackered, so i checked cambelt timing and was ok (thanks to the guys on here for showing me how) and all is good.