Hi, I've a similar problem with a DP55KG + i7 860.
I've overclocked my pross with 166 blck, 1.35vcore dynamic and turbo on. It's stable.
But when I try to wake up my pc, it frozes quickly. I was able to check the vcore before it frozes and the vcore seemed to be reset !
So I guess that's a wrong bios programmation with sleep mode and volatges...
I'm having the same issue with my DX58SO. There appears to be a BIOS bug that resets vcore everytime the computer enters S3 sleep mode. From what I've noticed, other voltages seem to be retained (qpi, vdimm, etc). For now, I'd imagine you'll either have to use S1 sleep or overclock your 920 as far as it will go using stock vcore (auto vcore setting). You may be able to coax a bit more out of your chip at stock vcore by raising vtt (qpi voltage), but it's far from an ideal solution to the problem. Hopefully they will release a BIOS update that fixes this terribly annoying bug.
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Please bear in mind that Intel does not provide assistance for overclocking. I would recommend waiting for another community member to post similar experience.