New to the forums, just bought a '91 Prelude with a VTEC H22A in it. I'm in japan, so the '91 is actually the same body style as the '92 in the states, if I'm not mistaken...the BB1?
Anyways, the car will idle fine at about 600-700 RPMs however the idle gets very erratic when I turn the AC on. I had the AC on and left the car running while I walked into a store(japan is awesome like that...no car thieves) and just as I'm walking back to my car, the engine dies.
I've also noticed that when I rev the engine lightly(up to 1500 or so) and let off the gas, the RPMs will sometimes drop as low as 100 RPMs and fluctuate before balancing out at 600-700 again. Any ideas as to what the deal is?

 

My 97 BB6 along with my friends 04 STi do this.

Neither of our cars has died on us, but they both idle weird when AC runs.