David, we too have this problem. I think that the idle stabilizer is sticking
on the central shaft.  The shaft is probably worn from constant movement
around the idle position. When the engine needs to rev higher - when cold or
when the A/C is on - then the stabilizer is holding the valve open wider,
outside of the worn zone, and isn't sticking. To test your IS valve, try
spraying a small amount of fine oil into the air intake of the IS valve
(engine off) and see if the symptoms change. Larry

> <snip>Then, I exchanged the idle stabilizer valve itself.
> Now, it idles when cold, but after driving even a little, I notice the
> idle speed going up and down in a regular fashion every couple of
> seconds. It doesn't go up a lot, but it's noticeable and new.  This is
> all with the AC off.  When the AC is turned on, the idle speed comes up a
> bit, and there seems to be less fluctuation though I still notice a
> tiny bit of the up and down.<snip>