It is a 2000 Cadillac Deville and the thermostat is replaced the radiator is good. So I do not know what it could be, some say it might be the head gaskets.

What is the symptom???

if the head gasket were going, you would notice one of the following: coolant dripping from the exaust at start-up, smoke from the exaust, murky or bubbly oil from coolant mixing with oil, oily, brown film on top of radiator fluid from oil mixing with coolant.

if it only overheats when sitting still/idling then the problem is your electronic fan is not switching on.

if it loses coolant only during cold weather, and you notice your heater smelling funny, then it could be a hole in your heater core.

Plastic impeller on the water pump and the plastic impeller is slipping on the metal drive shaft ??high possibility