2000 Cadillac STS • 100,000 miles

I have a cadillac sts 2000. The motor warms up after a while and the temperature over heat. The prestone overflow of the reserve tank. I have made check the head gasket to a dealer and its ok. I also changed the cap of the prestone tank and it still overheat. Tha termostat has been changed and I vacuumed the prestone under pressure. What's wrong, the dealers and mecanics can't find the problem
December 28, 2011.

Does the fan work properly? Has the cooling system been power flushed? The radiator may be clogged. The water pump may be not working.

Dec 28, 2011.