I spoke too soon again since it happened at about 8:48:57 AM PST for about a second. Now, I have a suspicion on my Debian/Linux's arpwatch process might be causing it but it just monitors? I noticed that it wasn't running a night ago so I manually started it, and my router rebooted again this morning.
Man says: arpwatch - keep track of ethernet/ip address pairings
$ apt-cache show arpwatch
Maintainer: KELEMEN Péter
Depends: adduser, libc6 (>= 2.7), libpcap0.8 (>= 0.9.8)
Description: Ethernet/FDDI station activity monitor
Arpwatch maintains a database of Ethernet MAC addresses seen on the
network, with their associated IP pairs. Alerts the system administrator
via e-mail if any change happens, such as new station/activity,
flip-flops, changed and re-used old addresses.
Tag: admin::monitoring, interface::daemon, mail::smtp, network::scanner, network::server, role::program, use::monitor
Or maybe just a coincident? I will keep this daemon running a few more days to see more reboots, and then turn it off to see if it still reboots.
1/16/2013: I saw another reboot this morning. For now, I disabled arpwatchd service to see if the problem returns again. To be continued...
1/18/2013: It happened again and with arpwatch disabled.