Intermittent Idle Problem, A/c Related Options

[Poonicus]

Alright so since about 3-6 months ago I've had a rough idle problem whenever the a/c is on, but whenever the a/c is off the car is perfect and this is Texas I kind of need my a/c on. Whenever the car is at running temp and even cold it idles fine at 800rpm but whenever you turn the a/c on it will go up to 1100 or so and then it will shoot down to 100-200rpm and the whole car shakes. I was wondering if this was because of incorrect belt tension, a vacuum leak, bad iac, or because that's what the celica's do when the a/c is running. Any help would be appreciated.

[VavAlephVav]

First I would watch the ac compressor while its running, does it "rattle" at all. It may be that there's extra friction in the compressor because it needs a little more oil added to it.
The ac compressor oil is in little cans at O'Reilly you can add it through the low side port with one of the small hose fittings they sell. If the compressor is noisy you just add a little more
Oil to it until it shuts up, probably less than 4oz.

Otherwise I would guest maybe there's a vacuum leak somewhere or the IAC is not working properly. But if it idles normally without the ac then the vacuum leak might be more likely, or something in the vacuum system.