About 5 years ago I took off very early on a 25 degree morning on a 250 mile trip to Santa Barbara. Half way there my heater suddenly began blowing cold air (???). About 20 miles further down the road I stopped for gas and when I resumed my trip my heater was blowing hot air again. That odd heater operation has bothered me ever since as I could never understand why it suddenly began blowing cold air that morning. Early this morning on a trip to Fresno my heater began blowing cold air again and I did some serious thinking about what could cause such a bizarre operation again. I turned my A/C/heater selector switch OFF for a minute and when I turned it back ON my heater began working normally again. It acted like the heater control valve somehow shut while I was driving at a steady speed and when I turned my selector switch OFF it relieved the vacuum in the heater control valve's diaphragm. Now I'm wondering if my 7 port selector switch is leaking internally; allowing a VERY slow vacuum buildup inside the heater control valve's diaphragm. As thermostats don't just "fall open" while you're driving at a steady speed and then recover an internally leaking selector switch makes a lot more sense. Do any of you have any other ideas about this bizarre heater operation?