I've got a 2600K with the Extreme4 board and I seem to be having some issues that are similar to what I'm hearing other people say here. When I first started overclocking this rig, I could get up to the 4.4 automatic turbo overclock in the UEFI to boot into Windows, but 4.6 just gave me the black screen with blinking cursor. I could get 4.5 to load windows by setting the multiplier myself, but 4.6 wouldn't boot Windows no matter what I tried.

Then, I came across a post by blacksun1942 that linked to some foreign site that had some settings that blacksun1942 said allowed him to get into Windows at 4.8. The link is http://ts2peca.no-ip.info/?p=604

When I used those same settings I was suddenly able to get into Windows at 4.8 as well, but any kind of stress test would either blue screen or freeze up the system. I wasn't willing to give it any more Vcore for stability so I backed the voltage down to 1.35v and was trying to get 4.6 stable in Prime95. I actually had it stable with small FFTs overnight for 8 hours without a single error. I then rebooted the system because I was going to try one notch down with the voltage so I could get better temps and decided to shut the system down completely to make sure it would cold boot properly because prior to using those settings I could never get 4.8 to boot Windows before. After shutting off the system completely and turning it back on, sure enough it was back to the black screen with a blinking cursor. Keep in mind I had changed nothing at this point from when it was 8 hours stable in Prime95 small FFT, but now it would not even boot Windows. It got past POST, but no further. This makes no sense to me. Since then I've tried every combination of settings I can think of and I can't get into Windows at anything higher than 4.5 even if I use the settings from that site again.

I'm no expert about this kind of thing, but it doesn't seem like a CPU limit to me. If it was the CPU, then why would it fail to load Windows at the exact settings where Prime95 had run for 8 hours previously without errors? I've considered doing an RMA of the board, but I'm hesitant because I don't know if it's actually the board or the CPU. I was hoping a future BIOS version might help but I just flashed 1.70 earlier this morning and nothing has changed.

At this point I'm not sure what to do. I've got 4.5 stable at 1.3v and the system is certainly fast enough, but the fact that I had it stable at a faster speed that I can no longer boot into is really bothering me.