Well, if an Offset CPU core voltage is being used, then both "CPU C3 Report" and "CPU C6 Report" have to be disabled (they may have different names than this, but "C3" and "C6" should always be a part of the name). They're just C-states that, when enabled, allow the core voltage to go down to extra low levels when the CPU is idling. So when overclocked, these have to be disabled so that the idle (or even low-usage) voltage doesn't dip down so low that BSODs and/or lock-ups occur. The voltage will still go down to nice and low levels, but having C3 and C6 disabled will make it so that it doesn't go down too low.
If this isn't the culprit, then I don't know what else to suspect (although, I am confident that someone else will know).