temperature contol problems 2003 prerunner V6 Ext cab white 2x

Last year I thought I was having problems with my AC. That is not it the problem is with the heat. If the AC is on, or if the vent is on, the heat will come on at full heat some times. To get the the heat back off you have to turn the heat control up and then return it to the cold position.
That usually cuts the heat back off. A few min latter the heat might come back on and you have to do it again. I do not know if the fan has to be running or not (in the vent only position). Any one had this problem? Do I need a new controller. Is there some special heater valve somewhere that might be going bad?

Karl

 

It seems that the problem is the same w or w/o the fan running.
Come on guys someone has seen this before!
Help!