I have a 2004 Pontiac Grand Prix whit overheating problems. The heat doesn't seem to be working, and it overheats when driven very long at all.

Is the car losing any coolant or does the fluid level remain OK? If it is OK, you may have a plugged radiator even though you think it has been "flushed".

Did you bleed all the air out of the system when you did the coolant flush?

I agree with Dorian. It sounds like air trapped in the cooling system. Modern cooling systems are difficult to fill without trapping air inside and sometimes even using the bleeder valve does not seem to do it. You may need to have it filled professionally by a shop with the proper vacuum equipment if you can't get it to fill properly yourself. Might be money well spent to get it mobile again.