The title describes my problem pretty well. I'm driving a manual transmission 2003 Toyota Corolla, and this happens whenever the weather starts getting cold.
The engine revs up fairly high, then goes back down, then back up, etc. The colder the car is, the faster the rate of revving is—until the car warms up. When the car is warmed up, the engine doesn't rev by itself anymore.
This only happens when the car is in neutral or when the clutch is pressed down.
I have no ideas, but some research has told me that it might be a vacuum hose?