@papatsonis:

I have that exact problem on my P8P67 PRO: it seems to be the way offset mode works, and it happens not to play well with high OCs. At stock speeds a 0.05V-0.1V Vcore difference between single and multiple-thread Turbo is perfectly fine, but at 4.5GHz and over it causes major instability. I haven't found a good solution so far, but trying multiple combinations of LLC, Offset, Additional Turbo Voltage, and C1/C3/C6 might help you get better results.