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It could be so many things. Do you hear the electric fan turn on? It could be as simple as a bad radiator cap, (allowing fluid to flow from the radiator to the expansion tank but not allow fluid to return from the expansion tank to the radiator as the system cools down). It could be a more serious problem like a mildly blown head gasket or a thermostat that does not fully open or sticks intermittently, another possibility is a radiator that is not working efficiently. It's too hard to guess with out being able to do some basic diagnostic tests or experience the problem first hand.