Could be the Throttle Position Switch on the side of the throttle body acting up, though I had a similar situation that started happening when I had a coolant leak and the temp would start to climb, in stop and go traffic.
If you put a load on the engine, do the rpms drop back to normal, such as turning your AC on? Or giving the gas pedal a small rev help it at all to drop down?
It may be a coolant leak or the tstat went out and stuck closed. As odd as that sounds, compared to the other ideas, that's was the reason my car had the same symptoms. My leak though was around the tstat housing to the head.
I also still had all the Cruise control crap on the car during that time as well.