Try checking for a Vacume leak in the brake booster,had this happen a couple of times.when you run your A/c you are putting a strain on the motor.If your booster is leaking when you apply the brakes it would be enuf to cause the motor to stall out.the booster is round black metal thing attached at the firewall beside the master cyl. when mine was leaking I would smell carbon dioxide inside the car while driving down the road.I beleive there is a diaphram inside this booster.also check the large vacume hose going to the booster