If voltage is fluctuating you need to isolate the problem. Make sure the PSU is able to handle the load at that voltage. Could be the VR on your board is failing too. I'm pretty sure mine isn't nearly as competent now as it used to be. If core is failing also raise the PL + 1 and see if that helps.

You think one core failing at high overclock is an issue! HAH! I'm getting a headache trying to find stable DRAM timing values on this DFI board heh. Playing with CLK/DQ drive strength at the moment because I'm getting instabilities few hours in. Also playing with DRAM DLL skew models, apparently DFI don't have the models for DDR2 listed anywhere so i'm playing guess who!