My vote... BIOS settings, RAM or PSU in that order. But as dmdilks said, if its only occasional, I wouldn't worry about it.
I was thinking the same, ruled out PSU. So definitely either a BIOS setting or RAM.
After enabling XMP I went about 10 days until it happened again. Oddly enough when it did happen, it was hours after a power outage (was at work), even more odd is that my PC is plugged into my UPS, and was off at the time (PC was off, UPS was on). It restarted during POST, then did it again, then said "Failed to Boot" (never said why), then it booted just fine.
Happened another time after that, I then enabled Fast Boot in the BIOS and will continue to keep an eye on it. Won't really worry about it, might try different RAM if the money is right. More so annoying than anything.
Also updated to F22 BIOS a few days ago.