What causes a 1985 Chevy Caprice to over heat?

I would first start with the cheap stuff first and then go up. I would start with the Thermostat. If the problem is still there after u changed it I would then look to the Radiator to see what kind of condition that it is in. (LOOK FOR RUST OR ANTIFREEZE JUST NOT FLOWING) Then I would check the water pump. With a car like the 85 Chevy it really cant be much wrong with it especially if it is running smooth.