I had this once...
I think it was due to a failed sync, (I might have pulled the plug of something, I can't remember) such that the database was not rebuilt properly.
When it reboots, it tries to rebuild the playlist, however the partitions were mounted read only, so it couldn't save the completed playlist.
That's from memory of the serial port debug, but it was a while ago, so I can't remember fully.. =)
A resync fixed it though.. That part I'm sure of... =)

Jazz
(List 112, S/N 00030, 4 gig blue)