When you first unlocked the fourth core BEFORE you flashed the BIOS did the fourth core stay unlocked when you re-booted? If so then why not revert to the original BIOS atleast you could get four cores. If it still didn't work then the only thing I can think of is maybe the fourth core just isn't stable and that's why it's acting weird. Remeber the fourth core was locked for a reason most likely because it was defective at the factory. Getting a core to unlock and be stable is the luck of the draw.