Two things to check... with the engine off, take the boot for the air intake off at the throttle body. Open the throttle plates by hand, and look to see if there's a build-up of carbon around the bores of the throttle body. If it is clean, then you may need a new IAC motor. This stands for Idle Air Control. It is an electrical device that fits into the throttle body and controls the idle speed by controlling the size of a passage in the throttle body. This will get carboned up and stick. The IAC motor requires a special procedure to install, so if you change it yourself, you'll need to look in a service manual to see what the exact procedure is. This is all assuming that the car is tuned port injection. If not, let me know.
[This message has been edited by Dave Birdwell (edited 05-04-99).]