I will check the radiator first, maybe is rusted or clogged with buildup on the inside, also a good clean of the outside fins check for leaves or dirt clogging air flow before moving to the inside inspection, if that is not it, next check should be the thermostat for the same reasons, and if that was not the problem either most likely will be the water pump, some water pumps made of metal lose their propellers due to corrosion.