Restricted radiator flow/bad thermostat are likely suspects.

Pull off the main hoses and check the flow with a water hose
and change the thermostat. If the radiator flow is'nt unrestricted
or you're not sure take it to a radiator shop ASAP.

You did'nt say what year, but if it's more than a few years old
the radiator shop might be the 1st thing to do - Overheating
engines have a way of turning into brand new engines, with a
wave of your wallet...
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