i suspect its not the air lock; if the rad is cold it means the thermostat is not openeing to allow the water to go to the radiator. Hence why the car is overheating as the water is just cycling round the head

change the temp sensor on the exhaust side of the head as a matter of course, cheap and easy to do although i think the size is 1/2 inch spanner

i;d also get a rad switch which fits to the bottom of the rad, then check the wiring; quite common for the wiring to mess up on the back of the rad switch