This is a common problem with Sandy Bridge systems, and they haven't found a fix for it yet. Mine does the same thing -- for a cold boot, you hit the power button and it comes on for a few seconds then shuts itself down for a few seconds then turns itself back on and boots normally. It only happens on cold boots, and only reboots itself once when it does happen. It happens on default settings as well as overclocked settings, so it's not system stability related.
This isn't a problem in my situation, as I very rarely turn my machine off -- maybe once a month or so.