Couldl be the water pump - Check if the water is being recirculated back into the header tank, take off the small top hose with the engine running

Check for fault codes see the sticky link at the top of the mk4 page "first thing to do if you car has a fault"
My money is on the sensor, i have just fixed a 1.4 Seat which had the same symptoms and it was the sensor - if it is there will be a fault code

Oh and buy the MK2
