Running the Heat OR the air conditioning will cause this car to overheat.

Ok, I will drain the radiator and change the thermostat. The Thermostat is in the lower hose.

correct. lower hose and 2 bolts. easy repair. make sure you properly bleed the system Roy

Ok I changed the Thermostat. Not so sure that I have "Properly bled the system" We took off the upper hose, and added as much antifreeze as we could. put it back We idled it for a long time until the lower hose got warm, with the radiator cap off. Then we topped off the antifreeze when it went down as the thermostat open. We also opened the air breather on the left hand side of the radiator, 1st time I may not have had the heater on. Air seemed to come from somewhere after 20 miles, the overflow tank overflowed again. The car will 2nd time, we did the same with the heat on to make sure that we didn't have any air in the heater. Worked fine for a long time yesterday. Idling a long time and a short trip, no problem. Today, my son went 20 miles again, and the temp gauge rose up and down a few times, and the coolant had been pushed out of the overflow tank. (The level was way down at the mark when the trip started.) So either we don't know some trick with this car, or we have another problem

you better get a leak down test for a bad head gasket. the only other suggestion is to replace the rad cap but it sounds like it is working. Roy

UPDATE We took this car to a mechanic who believes that there is a head gasket leak. He is trying a sealing additive that he had had good luck with. One day in a row...its good

temporary at best, if at all. the best repair is to replace the head gasket. Roy