I got a good feeling it is the motherboard. I am having the same Mobo but with Rev 1.0, and have been having sudden shutdowns about a month ago. I got a buddy who had the same board and hardware as we purchased at the same time, well, his mobo died 6 months earlier. It had sudden reboots happening until one day, it's dead. He did changed his PS about a year back as the first PS died.
My other friend told me that his friend had the same mobo but not sure of the Rev and it also have sudden shutdowns.
My sudden shutdowns occurs quite consistently about 5 to 15 minutes upon boot up. If it shuts down, occasionally it will self reboot. If not, I hv to manually turn off the mains power for about 15 secs before I can re-start the PC. Normally, after the sudden shut down or re-boots, it will be stable. My mobo is 3 years old in Oct 2009.