A couple of the simple reasons for overheating are, overflow hose is sucking air because of old age and very hard to detect, rarely do they leak fluid, but they suck air when cooling. Because of this, the overflow looks to have fluid in it, but the radiator itself is low, causing the overheating. With the engine cooled down, remove the radiator cap and top off the fluid. If it is low, that's the problem. I have had issues with the thermostat housing becoming corroded and slow leak there, usually just a little seeping and very minor leaking. Radiator cap itself not working properly, as in not holding proper pressure, can be a small issue, too. And also agree with the radiator fans not working properly.
