I've encountered the same problem since installing Final Fantasy XIV and am currently at a lost trying to figure out how to fix it. I've built my computer in March and did not have any problems with it, then I installed the Final Fantasy XIV beta last month and the computer started rebooting in the same way you are describing, except it never happened while playing the game. I can play the game for a couple of hours, then stop playing and maybe 30 minutes later it will reboot straight to the BIOS screen, with the only message in the event log being unexpected restart. It will also reboot if I don't play the game after a fresh power on.
This seems 100% random since I cannot reproduce it. I've run some stress tests and that doesn't seem to make it reboot, and it also seem to run fine in Safe mode. Tried disabling all start-up processes and non-windows services with MSConfig and it still crashes. When I initially got the problem with the beta, I formatted and the computer stopped rebooting and ran normally up to two days ago when I installed the Release version of the game.
I was wondering if changing your PSU fixed your problem, as I don't have any spare PSU around to test my system with.