I just bought a 2000 gmc savanna with 100000 miles. Doesn't overheat at idle or running @ 70. But when I take off from a stop the temp gauge goes up really fast and then comes down real fast. I took the muffler off to help with exhaust gasses, still has the converter. 4 inch pipe not here yet. I'm guessing maybe one of the thermostats may be sticking? Anyone else come across this?