I wonder if the problem is the water pump. If it is not circulating the water the car over heats and coolant overflows into the contain like it is supposed to and then gets sucked back in as the car cools. Is the oil level in the engine going down?
I supposed it could be a small amount of oil might go through the gasket into the coolant but I agree with you, why would that cause over heating?
Maybe it is good I am not an mechanic by trade.