To add to what's been said, within the bios on some mobos are options for turning different CPU options (cache, etc) on or off. Might want to reset your CMOS and start from scratch.
But, being that you bought a used CPU from some geeks, it might be hosed. Is the CPU unlocked (bridges closed)? Maybe it's been thru some extreme OCing and its Kentucky Fried Extra Crispy. :Q