Many thanks for the responses. Here is what happens: When the engine is started from cold, and the transmission is in "Park", the rpm goes up to around 1200 and stays there. Even when the engine has warmed up, the rpm remains steady at 1200. It is not until I put the transmission into gear, and load down the engine, that the revs drop down to around 900 rpm. After driving the car, and then coming to a stop, the revs will settle on 900 and sometimes will drop down to 800 rpm.....but never below 800. Turning the a/c on or off doesn't seem to make any appreciable difference.
It's my understanding that idle speed should be about 550 rpm. The last time I had the car to the dealer, the tech. noted the fast idle and guessed it was due to something electrical. Since then, I've learned it can be a host of things, just pick where you want to start.
I have removed the idle control valve for cleaning, so I don't know if it exhibits a vibration when the engine is running. I gather that if you can feel it vibrating, then at least the electrical part of it is working and the question then becomes: "did you clean it good enough?".
I think I now have enough info. to keep me busy for awhile........and I'm grateful to all that responded. Bill