I think it's heat. I had the same problem with a 622. The DVR is always working, making lots of heat. Mine wouldn't reboot itself like you've described, it just would start acting goofy and not respond. It would lose audio (optical output) on the off-air locals. After a reboot it was fine every time. I put it on it's own shelf, put a few blocks under it to raise it up, and bored a few holes in the entertainment center.
These units (622/722 DVR) run hot. Put your hand on it after a few hours. I see fan attachments for them on ebay.
Marconi.