heater problems

i bought a 1991 pontiac grand am a little while ago and i just replaced the thermastat and now my car heats only when it wants to. It may start out heating but as i go down the highway it turns cool.
I had the rad. flushed and they said i had a lot of air in it but they thought that they got it out. But i guess not because it is still doing the same thing it was before.
Can anyone help me with this stupid problem that i can't find a soultion and how do i get the air out the system????

The only way to get the air locks out is to make sure the resevoir tank is half full and crank the heat on full and run the engine. Turn it off, top up the tank, let the engine cool. Do it again and again and again. Sometime squeezing the upper/lower rad hoses helps a bit. It takes awhile. I changed the heater core and it took a week of driving to remove the air.
Don't open the rad cap...it lets air back in.