This is an odd one - every once in a while - averaging about once every 4-6 weeks - our P4 will be working fine and then just freeze, and usually slowly die and ultimately go into a reboot....or we will have to unplug it.
I think I've read on here before that manually rebooting occasionally is a good idea - though I never remember to do this. We keep ours on a UPS since our area has daily storms and tiny power outages almost as frequently.
Guess if I'm wondering if this is an ominous sign? We bought the extended warranty FWIW.