Got myself a 1.8T just recently (had it a week today actually) and I've noticed an odd thing.
The car warms up as it should, takes a few miles, sits at 90 degrees and that's where it stays. Until it decides to drop to the bump stop. The heater then goes lukewarm. There seems to be very little pressure in the expansion tank which makes me wonder if there's a blockage somewhere.
The pipe from the radiator to the engine gets hot, but the other pipes going to the expansion tank stay cold, with the water in the tank being lukewarm.
Any ideas? Worth taking the expansion tank pipework off and degreasing it? I have very little recent history with the car so I don't know if there's been any cooling issues lately. My usual mechanic is being all doom and gloom, telling me that someone must have taken the thermostat out and bodged the car, but I've now done 500 miles in it and there's no running issues (other than a squeal from somewhere, but that's another thread).