I think that the A/C is a matter of piecemeal troubleshooting. You've probably got that pegged already.

It sounds like someone misadjusted the throttle body at one point because of an idle problem, and therafter realized that the TB settings were all messed up, leading to high idle. They corrected that with the idle adjustment screw.

I have seen it a million times. The best way to get around that is to take a few minutes and clean out the entire throttle body. Here are a few tips from the school of hard knocks:

3 12mm nuts take it off
The plastic throttle rod clips are pretty brittle
clean out all of the holes that lead to vacuum or charcoal hoses too
perfect opportunity to check the intake hose for upcoming leaks or abrasions

If you don't have a manual that refers to the exact setting for the throttle body on that model, look at the 700/900 FAQ for more information. There is excellent information about doing exactly that. Chances are, you'll find it to be misadjusted. If not, it could be the idle air motor being sticky or sluggish, but the TB cleaning and adjustment is quick and free.


--
Chris Herbst
1992 745, 68k

And others:
93 944, 150k
90 245, 110k
88 744, 160k
87 245, 185k