2001 Ford Focus

With the air conditioning on the car runs fine when driving but when I stop at a stop light the engine bogs down every few seconds. If I simply turn the air off the problem goes away. I have replaced the surpintine belt but that has not fixed the problem. The car has actually died once or twice but usually just bogs down for a second and then goes right back up to the normal idle rpm's. What else might the problem be?