Not sure what part of the world you are in or your environment (home/work), and you don't mention what time of day the reboot occurs.
Why is this important? Well it might just be that you are in an industrial setting and there has recently been heavy plant installed that starts up at the same time each morning with a big glitch on the power supply. Big enough to be noticed as a temporary power loss.
I have experienced this, second hand, where a quarry suddenly began having problems and they thought the computer was on the fritz. A new bit of plant had been installed. It started up at 0600 every morning, causing the server to reboot. If the logs show nothing except the reboot then suspect memory or external power supply. I would expect memory to fail randomly because of a peak in workload but AC power supply because of interruption as explained. I have the T-shirt already. Two nights ago my SUSE box rebooted automatically three times in the night as I was using it, but the cause was due to the weather and external AC supply. This was a very abnormal happening. No UPS here either. I imagine from your name that the weather us a bit warmer than it is in the UK at the moment.
What time of day is the reboot happening and the weather at the time?
Last edited by PAix; 02-05-2009 at 02:13 PM.