I had a similar problem. The temp guage started climbing quite when sitting in a drive thru, so I let the car sit in a parking lot and cool off, then drove it home. The entire time I was moving the temp guage didn't really fluctuate, but as soon as I stopped in my garage at home, it started to climb so I shut it off.

The following morning I started it up and drove straight to the dealer. Again the needle stayed halfway (normal), until I stopped at the dealer and let the car sit at idle (started to climb so I shut it off).

It turned out that the hose going from the radiator cap into the reservoir bottle wasn't seated correctly in the bottle cap, so there was a ton of fluid in the reservoir, but none of it would go back into the radiator. I think I may have pulled on it by mistake when changing my oil the day before :dunno: