Ubuntu :: 9.04 64-bit Hard Lock At 7am?

Every once in a while I have noticed in the the morning that the computer is hard locked, meaning what ever was on the screen is still there (or just blank for screen saver), the machine is inaccessible over the network, and there is no kernel panic message in the logs.

This last time around my brother was logged in and he had the screen saver turned off, so I could see the exact time the clock stopped updating, and it read 7:00am. The var/log/messages log lists its last mark at 6:59:29.

A thought came to my mind the other month to check power stability because both my router and my network switch had a history of doing odd things even though I was using a line conditioner. After replacing the line conditioner with an APC Back-UPS XS 1500, both the network switch and the router have been perfectly behaved, but after a few weeks of continuous up time my Ubuntu system hard locked again and I don't know why.

Here are some hardware details:
m/b: Gigabyte GA-EX58-UD5
CPU: Intel 940
RAM: 12 GB @ 1066 MHz actual (1333 MHz rated, but can't do with banks fully populated.)
RAID: 3ware 9650 SATA-II 8-port w/ battery backup
Video: PNY nVidia 260 GTX core 216 based card with 896 MBs RAM.
P/S: Seasonic S12 650W (80Plus certified)