Two days ago, my girlfriend was driving our Millenia ('02, 2.5L) back to our house about 50 miles on the highway. About half way home she saw the temp gauge go up to Hot. She turned the heat on full blast and it came down into a more respectable range.

Today, I did some driving with it. It doesn't overheat at low rpms (town driving) as long as the heat is full blast. Once I hit the highway, or accelerated town driving, it creeps all the way up to hot. If I come to a light, or pull over and let it idle for a few minutes, it will slowly come all the way back down into normal operating range. If I continue to let the car run, as long as the heat is on, it never goes up into the hot range.

So here's a summary of what I know:

* Fan is kicking in, no problems there.
* Car overheats at higher rpms
* I replaced the radiator cap with a new one (no difference)
* Upper hose is hot, lower hose is cool
* Nothing is leaking, Coolant isn't low.
* If I rev the engine above 1500 rpm, temp gauge will go to hot eventually, even with heat
blasting

Could this still be the thermostat? I picked one up but it looks like more work than I want to do in this cold weather. Any other ideas? Water pump, belt, radiator? I'm hoping it is still the thermostat, maybe stuck half way?

Anyways, any ideas would be greatly appreciated before I drop it off at the local garage tomorrow.

Thanks,

Jason