2005 Subaru WRX high idle

The past two times I’ve driven to work I’ve hit significant traffic. While in the middle of stop and go traffic, the engine started to idle at high RPM (3000-4000). Today the temperature gauge really started climbing and hit the red, at which point I shut the car off and waited a few minutes. No smoke, no noise and no check engine lights. As soon as the road started to clear and we were moving again, the car was fine. The engine cooled, stayed cool, and the idle returned to it’s normal 1000 RPM.

The dealer was unfortunately unable to reproduce these findings, though they thought it could be a stuck thermostat. Anyone else have any ideas before I start replacing parts?

If the dealer checked the coooling system oevr rather than just attempting to duplicate the problem, which is what he should have done, than I too would recommend changin the T-stat. With all new coolant, of course. The correct coolant.