Ford Repair: Rolling idle, pvc valve, ford expedition

Question
I have a 2000 Ford Expedition 4x4 with the 4.6 v-8 it has appx 80K miles on it.  I live in Arizona so my air is on most of the time and while at a stop light in drive and the air on it will go from 800rpm to 500rpm then up to 800 down to 400 and so on.  It has never died and when the air is turned off or you go to neutral it stops.  It has been doing this for a long time and never gotton worse, I have had it to Ford and they replaced a pvc valve and treated the gas for the injectors and cleaned the throttle body.  As soon as I left and the engine warmed up it started again, they are stumped and wanted to replace more parts without knowing what it was.  I replaced the idle air control sensor, didn't work either.  Any ideas?  Your time on this is greatly appreciated.
Thanks,
Dave

Answer
First of all, I suggest you try a different shop. Replacing parts indiscriminently is not the procedure for fixing anything. Has the check engine light come on at all? What you describe sounds like the AC clutch off/on changing the idle speed. The AC creates more load on the engine thus raising the idle speed. This may be normal. There are ways to observe the actuators and sensors to determine if anything is out of spec and not setting a code. The idle control valve has been a problem on many Fords, but if yours is changing the idle it should be ok. Most times they will stick instead of changing. This valve only does what the computer tells it to do. I have not heard of any reprogramming updates, and found no TSBs related to this idle concern.