i sort of agree with the 2nd post, just for different reason. ac takes power from engine, engine is old, pcm has to adjust by adding more power to the engine, my guess would be - by sending more air/fuel into it, revs go up, ac turns off - it all goes to normal, and so it goes.
i don't think there's anything wrong with the car. it's called AGE. i just turn the screw on idle limiter to jag it up higher some.