So I've had my car serviced for this problem a couple times with my local mechanic, but they've never been able to duplicate the problem. Here's what happens:
All gauges appear normal, and check engine light has not lit up. But when I'm stuck in traffic the idle speed creeps up with no apparent limit. It's worse in warm whether. It's gotten as bad as 4-5k rpm, at that point I do whatever I can to get off the road and either turn the car off, or drive someplace without traffic.
When I get out of traffic, and drive normally for 10 minutes the idle goes back to normal. Or, if I turn the car off for an extended period of time, the idle goes back to normal.
This is a 2005 Lancer ES with manual transmission.