It happens very intermittently, and only when it's idling at a stop, never while driving. It can happen twice in a day, and then not for two weeks. And it starts right up again. Haven't taken it to a dealer yet, wanted to see if others had seen the same problem first.

Update from Nov 15, 2011: Believe it or not, it was the alternator. Replaced it and hasn't acted up since. It must have had an intermittent inside.