2003 Grand Prix heater heater issues

The heater blows mostly cold air, sometimes in town will act like it going to start blowing warm air then goes cold again. On the highway as long as you got the gas pedal depressed, it will blow warm air but never "hot". The second you get off the highway it goes cold and even when you take your foot off the gas (on the highway) seems to cool down. Temperature gauge typically at 175-180 unless you stop and/or turn if off and may rise to 190 but still no increase in heat (air temp was mid 30s). Upper heater hose when idling stays cold while the lower hose gets hot.

Anyone have any suggestion, your responses appreciated in advance.

Ken

 

Faulty thermostat perhaps or maybe plugged heater core

 

air lock in the heater core, or low on coolant, low on coolant is the most likely thing.

life begins when the kids leave home and the dog dies.