96 olds cutlass heater problem

I had replaced the head gaskets in car and put in new thermostat. I bleed the coolant pipe near the water pump. I must have missed something as I run the car the heat gauge moves up one fourth of the way, but what confuses me is that when I idle the car the heater blows cool air or lukewarm air, and when I push on the gas to make the motor run faster the air turns much warmer (like it should be) and when I let the car just idle again the air turns lukewarm again and even cool.

Any help would be appreciated.

Thanks

it sounds like either air or a low level.there should also be a bleeder around thermostat housing.did this start after the head gasket or was it a problem before?

I checked out that 2nd bleeder which I was not aware of, and that did the trick. heater works great now. Thanks for your help- much appreciated!!