Erratic Heating

[JDC55]

Hello all. I have an issue that just recently popped up. I noticed that at my usual 71 degree wintertime temp setting, I stopped getting heat. If I crank it up to 78 or so, I can hear the actuator open and then I have heat, for a while. If I go to 80, same thing. The Interior Sensor Fan at the side vent is working fine, but can the sensor itself go bad? Is it possibly one of the other temp sensors somewhere in the system? All the actuators are operating, so it's got to be somewhere else.

I'd be grateful for any ideas.

[wvicary]

When you have heat is it hot then slowly goes cold or just warm? I assume the engine temp gauge and coolant level are normal.

[JDC55]

Yes, it initially is hot and you can hear the actuator changing, and it goes cold.