overheating

I have a 1998 grand am
problem: overheating

Ok so heres whats happening. The heater whenever on sometimes works when it should but alot fo the times not. When it is turned on but not working it blows out cold air and my car overheats. when its off it cools down again. just wondering what the problem could be.

my friend said it could be the water pump. Any opinions are greatly appreciated.

Top off the coolant and bleed any air out of the system and try it. If no good replace the thermostat. If still no worky the water pump is probably on the fritz...

Also if you have a 4 cyl if the heater core is pluged it can affect the flow of coolant.