Eratic engine speed
 

The engine in my '99 Trooper (3.5L OHC, std. trans) has for some time now intermittently raced (up to 2,000 or 2,500 RPM) and stayed at this speed for several seconds, then the speed decreases. Alternatively, sometimes when I disengage the clutch, the engine will just stall altogether.
I just put the computer on the OBD2 port and got a clean bill of health, so I'm puzzled. It can be dangerous, especially when it stalls because if I am turning a corner and downshifting, the stalled engine eliminates the power steering and it can get dicey.

Any ideas or tips?
Thanks!