Over the past few days it'* been biterly cold in MI. Perfect time to test out another theory on this nusance (sorry Bill, I just can't accept what you suggest...I firmly believe the Rs should come down on their own!).
SEEBonnie.... Try this. Before you start the car on a cold morning, put the HVAC selector on the Floor and Defrost setting.
After 2-4 minutes of high reving at 1500 RPM, the RPMs dropped to 950 RPMs in my 99. This is the only way I've determined to "force" the engine to drop the RPMs.