A thermostat stuck closed could cause an overheating condition. Also if the coolant is too low it could run hot. To purge the air just run the engine for 15 minutes or so with the radiator cap off and keeping the level as full as possible. Other possiblities are.....radiator-over time the coolant flow is reduced thru the core and can cause overheating. Also if the headgasket is leaking "combustion gas" into a coolant passage overheating will also be the result. A chemical test for carbon monoxide gas in the vapor from radiator can detect a leaking gasket.