i had recently fixed my major overheating problem by replacing the half torn off air dam with a new one and it ran great for three weeks never got above 200. then randomly one night the heat stopped working and the temp shot to 240-260 then five miniutes later heat kicked back on and temp started dropping back to normal leveled out again at 200 and that was it for that night then about two days later it did it again on the interstate i was crusing at 100MPH (yes i was speeding) and it once again the heat stopped working and it shot to 260 i started to get off the freeway about 7 miniutes later as i was pulling into a gas station it shot back down to 220 then gradually decreased again. so i got back on the freeway after checking the fluid levels and what not and 20 miniutes later same thing then quickly as it went up it went back down. i suspect the thermostat but why i am reluctant is because i had replaced the entire system from pump to radiator plus all new hoses the entire cooling system is new there should be no reason for it to do this so i am asking for a second opinion. also when it is run real hard suddenly it will over heat then level off pretty quick and will slowly rise after getting off the freeway then once again level back off. it is actually starting to get more and more frequent almost to the point i can't drive the car (which i have to since my truk is down at the moment)

 

How long ago has it been since you did all the cooling system work?

When re-filling the cooling system did you run the engine with the cap off until the thermostat opened a few times to allow trapped air to escape?

 

its been about two months and it had ran great up untill this started. when i re-filled it i burped it at both the pump and radiator.

 

I'd bet it's a head gasket.

 

That or there's junk inside that will occasionally block the flow and then break free