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1995 Taurus temperature problem

95 Taurus, 3.8L v-6 with 150,000 miles. Soon as we start using heater water temp (per gauge) becomes very erratic. Close to overheating. Turn off heater and temp goes to normal (usually). Not losing much water, no sign of heater core leak. Is this likely a clogged or failed heater core or is there another likely problem?

How Much Would You Describe “Not Losing Much” Coolant To Be?
Your coolant loss should be almost imperceptible during a several week period.
You could have a leak, bad gasket, or air trapped in the cooling system. Has any part of the cooling system been opened for service, lately?

Assuming you have heat in the car the heater core is not clogged. How much coolant, exactly, does this car lose, and is there a visible cooling system leak? The heater core is normally full of hot water. When you turn on the heat all you’re really doing is directing the incoming airflow through the already-hot core.

I think the problem could be electrical. Does this problem happen in conjunction with turning on the blower (fan), or does it happen when you turn the temperature control knob?