my guess:
maf sensor. its the lil wire (that runs to a black box) connected to the airbox. my car (5spd) sitting out of gear has that problem sometimes, revs going from ~1200 to 400 and just waving around. it does it severly enough to stall the car jsut sitting there. my wire is obviously getting a bad connection and i can see that. yours my be the connection or the sensor itself. as far as a/c problem, i know it raises revs but not that much, and i dunno if its linked to the maf because i have no a/c