lisapivaral

I have a 2000 Chevy Cavalier and I love my car but i am having some problems with it over heating. I put a new thermostat on it, a new radiator bottle,but it is still over heating. I had it checked for a blown head gasket but there ok. could any on help me out a bit? Do you think If I have it flushed it would be ok?. It only over heats when it needs coolant i have lots of coolant whats the problem?