My girlfriend experienced the same prob with her 8v (soon to be vr6 hehe)
anyhow after a lot of unsuccessful probing for the prob, and with the help of vag com it turned out to be the lamba, when we replaced it the prob disappeared,
it almost felt like the car was dropping a cylinder, no power or acceleration, then it would clear itself for a bit, then reappear again etc etc... Lamba was the culprit...