Thanks for the advise.
But today, I have the reverse problem.
It was cold outside this morning (about 36degF). Once the car had reached it's normal operating temperature , the heater was functionning perfectly. The inside of the car quickly reached a comfortable temperature.
Cruising at 70MPH, the climate control switched very rapidely to very cold, as if the A/C was put at max. About 45 sec. later, it switched back to heat and temperature stabalized.
This pronlem seems to be ''vacuum ralated". It switched to cold when decelarating (coasting to about 55MPH) and switched back to heat when I resumed speed with the cruise control.