It wouldnt go higher than 4.2ghz at 1.25v. So I set it to 1.35v and 46 multiplier. Now the pc rig I've just spent $$$ building won't even go into BIOS. "Machine Check Error", then hard reset (all lights/fans go off for 5 seconds then restart, me pressing reset or power does nothing) or Flashing cursor... I have to press reset myself. I've simply switched off my entire rig by the psu and taking a breather.