The heater in my GTi reaches temprature in about 4 minutes which is great but when the engine reaches running temprature the heater suddenly runs cold and then stays cold untill the engine is left off for about an Hour. I have checked for loss of water or head gasket leakage into the water or water into the oil and there is nothing. It is a realy rapid cooling from hot to cold in 20 secs once the car is at running temp. Any one got any ideas an air lock would surely be there all the time???