My problem also has the random freezes, and boot cycling, as noted by horrorwood.
Is anyone who is having these issues NOT using an i5 2500k?
I also have an i3 2100, and I'm going to toss that in there for testing. I actually had to go out and buy parts for a 2nd system because my primary system is so unstable, I cannot trust it for my daily work. I am beyond annoyed at this point. I work full time in IT - I should not be working for free trying to troubleshoot issues for my own PC build. Sorry /endrant.
So thus far I've tried 2 different power supplies. 4 different types of ram (all pass memtest). The system actually passed a prime95 burn in, but then it will sporadically lock up while sitting idle, doing nothing (while I am reading web pages in between scrolling). I've tried 2 different Video cards, and SLI vs Single (although all cards have been Nvidia - 460gtx sli / 8800gt). A different Hard Drive versus 2 SLI SATA setups, different DVD-Rom. Thus far, all things are pointing at the board, the processor, or a combination of how the two play together.
I've reinstalled Vista 64, and Windows 7 x64 so many times now, that Microsoft has to be wondering what's going on with my Keys. I just hope this extra information helps.
Like I said, if Gigabyte wants my system for testing, please send me something comparable that works. I'd be more than happy to submit a sample so they can get to the bottom of the issue. I have a feeling this is a larger issue than we think it is. I'm betting most are just dealing with the "boot cycling" think its user-related - ie I screwed up my windows install or bios settings somehow. However... it cycles with default settings.