if your AGU has developed a missfire (usually on on cylinder 3) the ignition module is a very likely culprit.
The early 1.8T's don't have a management light and this fault doesn't usually throw a fault code. The coilpacks on an AGU are rock solid because they don't have amps built into each coil like the newer 1.8t's (AUM, AUQ).
For the AGU this is controlled by one ignition module.
As a first step :
Make sure the coil packs, the wiring to them and the spark plugs are all ok (Swap the coilpacks and spark plugs to see it the missfire moves to another cylinder)
If not it could well be the ignition amp AKA Ignition Control Module, this is located on the airbox at the rear.
Two screws and just unplug and replace.
Ignition Control Module, part numbers Bosch 0227100211 or 4D0905351.
Eurocarparts list these as "TCI MODULE" (Transistor Controlled Ignition) part no 415440120
You can get one from Euro car parts and use the forum discount
http://www.eurocarparts.com/ignition-modules£66.60 with forum discount
http://www.golfgtiforum.co.uk/index.php?topic=206325.0http://www.gsfcarparts.com/925vg0350
