I've lost a few belts at the track. Common cause seems to just be the belt and not any misalignment of any of the driven components. Everytime I've had the same problem, I put a new belt on and its good for quiet a while. I recommend a new belt for you. But, when the belt is off, I'd check the driven components, such as the alternator, for excessive longitudinal play (e.g. in line with the shaft which would cause it to be out of line with the belt).
