I went through the same pain with all of mine - i3, i5 and i7. It took a long time to crack it but the last 50mhz is always the hardest. Try messing with LLC and raising/lowering bios vcore to match load voltages. I always run LLC completely at intel stock for the last bit of stability - however asus boards behave somewhat differently so just experiment a bit. P8P67-Pro has made me break that rule.
Its probably worth taking a step back and finding the nearest stable setting and work from there. Diff bclk/multi maybe?
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