ac blows hot and cold air

I have a 2002 Passat with climate control. When the ac should be blowing cold air, sometimes it starts blowing hot air. Eventually, it will start blowing cold air again. The car does not have to be running for any certain amount of time, it could happen immediately. I have had the car in to the dealer, but they could not find a problem. Lowering the temperature setting, turning the ac off and on, changing fan speeds, etc. does not change it back to cold air. It just eventually returns to normal on it's own. Does anyone have any ideas?