This sounds like your idle stabilizer valve is getting sticky/worn happens to all of them sooner or later. When your A/C is engaged the engine needs more air (power) so the idle stabilizer is opening more with A/C , off is drops back to the lower range where it has been working for the last 20 years or so. Your fuel injection computer brain is watching and adjusting the idle speed all the time so it SHOULD be trying to adapt to any running isues but when the stabilizer valve goes bad the brain is helpless with no way to correct the idle speed.

 

Jim,
thanks,

Idle stabilizer valve was just replaced.
Big Dave is this valve the #12 unit with three hoses?
Do all three need to be plugged?

Thanks Rich