I havnt read all the posts.. but go to autozone or napa or 1 of those stores. There is a test kit they offer that tests for exhaust gas in your coolant. ( the name of the kit escapes me right now) but you put this blue liquid in the kit, put in on where your radiator cap goes and crank the car, let it idle for a few minutes. Your car sounds like it is doing the same thing mine was when i had a headgasket seeping. It doesnt run hot all the time. Hell mine would go a couple weeks without getting hot, then 1 day on the way to get milk or somthing stuipid, it would just run hot with no warning. You dont always have to have milky oil, or steam from your exhaust... these mustangs are gay sometimes. let us know how it goes man.