I've had a similar problem before where the engine would idle at irregular rpm's, either very low at ~600rpm or very high at ~1500rpm. It would often happen when shifting from drive to neutral. The problem was a stuck "Idle Air Regulator" or something like that.

As a note, the service technician was very unhelpful in diagnosing the problem and it took a little bit of convincing to get his manager who guessed it was the IAR. Perhaps this is a rare problem or just hard to diagnose since the idle speed was usually nominal...

Good luck. Let us know how it goes.