Was it after some repair / maintenance work on the car? If yes, start by checking around the areas touched during the repair for any loose connections.
Is the problem constant or intermittent? If intermittent it could indicate a loose connection as well.
I grappled with this same problem for a while, along with other related issues such as difficulty starting in the morning, stalling at stop lights, poor gas mileage etc. It turned out to be the wiring of the Throttle Valve Actuator.
If you search in these forums you'll find a whole lot written about this, and how you can re-wire the Throttle Valve Actuator - if it is the offending part that is (the price of a new or even S/H one is rather annoying)
Actually the Actuator itself rarely goes bad, the biggest culprit is almost always the wiring into it. If you remove it and inspect the wires inside you're likely to find the insulation all cracked and flaky, and it's all to do with the quality of material used in the manufacture (I hear they wanted it bio-degradable) Should this be the case, you may want to renew the Actuator or alternatively have it re-wired (with real automotive-quality stuff)
However, with the limited description you've given of the problem it would be difficult to categorically lay blame on the Throttle Valve Actuator wiring exclusively.
Maybe you could start off by checking the main Engine Wiring Harness for any visible signs of degradation / disintegration of the insulation (again this was a major issue with your model year). Try gently twisting/bending/pulling/pushing on the visible cables and connectors of the engine harness and the actuator while listening for any changes in the idling speed. (CONSTANTLY KEEP IN MIND THAT WITH THE ENGINE RUNNING, THERE IS DANGER OF ELECTRIC SHOCK, BURNS AND OTHER INJURY - SO BE CAREFUL)
By the way the Actuator's connector is a very strong animal, and so is the outer insulation of the cable connecting into it, so it could deceive you with its looks. The wires running in it are the problem. Bio-degradable
Please post back with your findings.