I have the Gigabyte GA-X79-UD5 motherboard with an Intel i7 3960X processor. 95% of the time when I try to overclock in the BIOS, it fails when I save and reboot and it boot loops without even POSTing. I'm currently running the latest BIOS version of F8, but I just flashed that yesterday; I was using stock until then. I've read that the F7 revision fixed some overclocking issues, but later versions like F8 include the stuff in prior ones to my knowledge.
Let me begin by saying that I'm a relatively seasoned overclocker. I've never been into it really hardcore, and I'm not an expert at it; but I did short some pins to overclock my processor like 9 years ago.
I like squeezing extra juice where I can. My 2500k Sandy Bridge sitting next to me is overclocked to 4.4GHz on air cooling and stable.
Now, once in a while, an overclock will stick. I see the proper GHz in CPU-Z in Windows with it and it's stable. I managed at one point to OC my 3960X to 4.3GHz with a 1.40 vcore and it was stable for 2 straight hours of Battlefield 3 without ever seeing a CPU temp over 62 during the gaming session. I rebooted, got the boot loop and couldn't POST, and had to reset the BIOS. I just submitted a ticket to Gigabyte a little bit ago with this info and more, but I figured I'd post on the official forums as well and maybe I'll get lucky.
Here's the real kicker. I can cause this same problem to happen without even overclocking. If I set the real-time CPU ratio control in OS setting to enabled, I get the same problem; boot loop without POSTing. (By the way, when I say boot looping, I mean before it ever has a chance to start loading Windows; 10-15 seconds after hitting the power button.) If I try turning off hyper-threading, the same thing happens. This is not just an issue related to overclocking, though that's where it's most prevalent.
Anybody have any suggestions? I'll try virtually anything. Also, if I hit the OC button on the back of the motherboard, the same thing happens.