My 2000 ZX2 auto with 120K . If the motor is cold I turn the AC on and it is fine but if it is at normal operating temp it kills the car ideling but it restarts fine. It I cruise down the road @ 1500rpms and turn it on it is fine?
I'm thinking you have a bit of a problem with the IAC. When you turn the A/C on it sends a message to the computer to bump the idle speed up a bit to compensate for the extra drag on the engine. When the engine is cold its operating under different parameters that don't allow the A/C to affect the idle speed so much.
It's also possible that there's a bad wire connection somewhere in the system that's not allowing the information that the A/C is on to reach to computer, so it doesn't know to compensate with a higher idle. When you're at cruising speed, it doesn't really matter. The engine's making enough power to compensate for the added load.