my coupe was doing this same thing it was baffling me for some time sometimes it would run cool but if i got on it or drove it on the highway for lenthy periods it would start to rise to 250+ i replaced head gaskets changed my intake changed to a cooler thermostat tried no thermostat you name it i tried it but i couldn't figure it out finally i discovered 2 things 1 time i remember getting on it hard and the car puffed a big cloud of white smoke (antifreeze burning) but it never did it again well when i was changing the intake from a downs ford upper/stage 3 ported truck lower to a tfs track heat intake i noticed upon starting the car it was smoking and everytime i revved it hard it would start puffing white smoke i thought i had just got a little coolant in the head until i noticed it was blowing coolant out the exhaust my first thought was a blown head gasket but not so i thought the head gasket looked fine i checked the head out and i couldn't find a crack anywhere i even took the valves out and checked inside the chambers to no avail so i put it back together and the sob was still doing the same thing turned out to be a tiny microscopic hairline crack in the #6 intake chamber when i was driving around i never noticed it blowing the coolant out the exhaust and for some reason after changing the intake and putting a 180 thermostat in it now it runs cool i didn't pull the head back off to fix it i just used some of that handy dandy coolant system stop leak and that fixed it and now thanks to my continuing driving of the car when it was heating up it has lost oil pressure due to eating up the main bearings i can't win for losing but at least it runs cool now.
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