440 overheats once and then it's happy all day

My guess is what El5demon340 said, and you have some air in the system.

Did this start occuring after something in the cooling system changed? When you refilled the radiator, did you have the front elivated so the system could be properly 'burped'?

My guess is that there is air in the system, and it's getting caught at the thermostat. The air pocket is acting as an insulator, and while the coolant is reaching temp, the air is taking much longer. When the air reaches temp, the thermostat opens an you are good.

Drilling a small hole in the thermostat will allow the air to pass thru the thermostat, but still hold back the coolant enough so it reaches temperature - remember the purpose of a thermostat is to get the coolant/engine hot quickly, not to keep it cool, so the hole should be as small as possible to allow the air thru, but not too much coolant.

I use a funnel like this to fill my radiators, that way I don't have to worry about raising the radiator when filling, or developing air pockets. http://www.rakuten.com/prod/complet...s&adid=18172&gclid=CMmVk_7b2L0CFZRr7AodZhYAnQ