Usually overheating – not associated with visible leaks – is caused by the electric fan not spinning nor spinning too slow, a faulty fuel pump or drive belt, an air pocket in the cooling system, a bad or incorrectly installed thermostat, a clogged radiator or faulty cap, or low coolant levels. If you’ve eliminated those, the next place I’d look probably is the engine timing and engine fuel/air mixture. Something wrong with either of those can cause overheating. If that leads nowhere, I’d ask for a leakdown test to verify the condition of the head gasket.