Yesterday while driving to work, the 240 temp gauge started heading into the red zone. It happened fast - probably took fifteen seconds to go from normal to full overheat (well above red zone). After that, it fluctuated between them. There's a pattern: when I'm accelerating, the temp drops; when I'm off the accelerator, it rises, even when the car's not slowing down i.e. on hills. The pattern doesn't hold when I shift into park and rev or idle the engine. It's got plenty of oil and water, doesn't seem to be losing any.

Could it actually heat up that fast? And cool just as fast? Does this vehicle have one of those problematic compensation boards?

Thanks, you guys are always good for getting out of a jam.

Alex