I have had this fault on a MK3 AGG 8v before.
I know the car's and engines are different. Check your Lambda Sensor for faults, mine was knackard so replaced it and although yours might be fine.
I finally solved mine 2-3 weeks after the sensor was changed, i found a large split in the vacuum pipe coming from the master cylinder.
I dont know if mk4's have this in the same place or not, if it is its dead easy to find.
My MK3 worked faultlessly after fixing these 2 right upto an usinsured tosspot killing it!
