High Idle Problem

1993 Sable... Feels like the tranny is slipping while driving, and when Put into Park (at a stop) Idle speed runs up to 4000 rpm's. Then hard to put back into drive due to High rpm's.... any Ideas?

1993 Sable... Feels like the tranny is slipping while driving, and when Put into Park (at a stop) Idle speed runs up to 4000 rpm's. Then hard to put back into drive due to High rpm's.... any Ideas?

Is the care throwing any codes (service engine lite on)?
Sounds like its the Idle Air Control Valve, or some other senser like the Intake Air Temp sensor, but without more info, its really a crap shoot.
Does it do it all the time, both cold and warm?