if still underwarrenty demand to get problem sorted once and for all, if not and vw cant solve it take on a rolling road and have any 'fault' codes etc looked at whilst under load, things behave differently when under load, they may have a better insite as to whats causing the probs as may not be happening stood idiling in workshop!!, normal faults are the coilpacks and mafs but you should get a reading for these being faulty (even though youve had changed) ive read of people having new coil packs fitted for them to go nearly straight away after!!