It takes a significant amount of power to run an AC unit, so it can stall the motor if your efi system is unable to compensate like stevebfl suggested. Power steering can stall vehicles as well, turn the wheel all the way to one side and you should see a drop in idle speed as the PS pump draws power trying to turn the wheels.

I wish I could help more, but I am completely ignorant on the details of your particular system. I hope you find an inexpensive cure...