no heat

I have a 99 pontiac grand am. That is not warming the inside of the car up when it is sitting idle. The only heat that I get is when I press the acelerator, then the warm are blows like mad, but once I let off it goes back to the same..blowing cold air. I changed the thermostat already and I am thinking that it is maybe the heater core.

any thoughts ...?

could be that the heater core is clogged or a weak water pump. That also would make sense too due the higher RPM's will circulate the better water.

air pockets, bleed air from the cooling system

That is one thing that I forget, is the air pockets.