Ultimeas

Car runs and pulls great until I turn on either the A/C or the defroster (obviously because its using the A/C).

The A/C system is fine. Full with no leaks, everything working great. When it idles with either of them on, about half the time the RPMs will drop then soar, drop then soar, etc.

My first thought was the IACV, so I replaced that, and it still does it. The weird part is, it only does it about half the time.

Also, which I guess is a common problem with new edges, I'll be coming to an off ramp off the highway, push the clutch in and use the brake, and the car will stall, which is pretty scary around turns with no power steering or brake booster.

Any ideas?

cliffyk

None of what you describe is "common with new edges", but rather common to 11-year old cars in general; that said there is something wrong.

When you installed the new IAC did you disconnect the battery for 5-7 minutes to clear the KAM and force the PCM to learn new idle air trim values?

You may also want to look in to re-adjusting the throttle plate stop screw. With age the contact points at the screw tip and the bell crank blade can wear causing the throttle to close a bit tighter--this will affect the {CM's ability to control the idle via the IAC,