[wolfman]

This assumes the speedometer is actually working, and that the SES (Service Engine Soon) light is NOT on.

The IAC valve is sticking OR the TPS (Throttle Position Sensor) is bad.

To test the TPS. With the car warmed up and idling, rap on the side of the TPS with the plastic handle of a screw driver. If the idle speed alters AT ALL, REPLACE the TPS. If it does not, the IAC (Idle Air Control) valve sounds like it is sticking SHUT. Remove and clean the IAC valve and its pasage, replace the "O" ring. If this does not improve things, the IAC valave probably needs to be replaced, or there may be a problem with its electrical connector.