2003 chevy impala over heating?

Is there goop under the oil fill cap , is the exhaust white ? That would point to a bad head gasket so then you do a compression test or block test . you should have tested the thermostat when you had it out to help find the actual problem , If your car over heats see if fans come on when it does , check coolant level and see if you have heat , replace thermostat and check old one in pan of water with a thermometer a bad one can make your temp gauge bounce a round , if heat is sporadic bleed system through highest hose on motor ,