My Droid X spontaneously reboots when it's playing music. I've confirmed this behavior with the native music player, Amazon's MP3 player, and now Spotify (though, with Spotify, it seems to happen less often).
No warning, and no pattern, the phone just goes silent mid-song, then reboots. Sometimes it does it on consecutive days, sometimes it doesn't do it for a week.
It's perfectly stable at all other times, and has never spontaneously rebooted outside of when I'm playing music.
I think it was a memory issue. I installed Auto Memory Manager, set it on "mild" and never had the problem again.