Heater blows hot when set to cold!!
 

I have just purchased a '99 Voyager SE which has an odd fault...
The heater (not air con) when set to cold occasionally starts blowing very hot air. There doesn't seem to be anything particular that triggers it. If I flick the slider to hot and back to cold again it will go back to blowing cold air.
Anyone else had the same problem or any suggestions as to what might solve the problem?