How much have you overclocked it?
These chips wont go much higher than 3.9ghz - 4ghz
Try upping the multiplier once and then changing the voltage by increments of 0.2 volts to find a stable sweetspot. then if its stable a decent voltage, check temps and if there is headroom, up the multiplier again continue tweaking the voltage.
Bear in mind im used to Intel chips for OCing.