Leon R

^^ exactly what I had too on my Mk2. Clean out everything with brake cleaner and if the pipe from the crankcase breather (from the front of the engine block running to the side of the airbox) is soft, change it. This happens also if the oil is overfilled past the max mark.

I would check the ISV is getting power using a multimeter, but as it's running ok after keeping the revs up for a while the coolant temp sensor could be faulty, so check the resistance of that.