The Sabertooth actually overclocks the processor by default, lol... turn all of the overclocking options off.
So reset CMOS.
Press F1 to enter the BIOS.
Change to Advanced Mode.
Go to AI Tweeker, and set AI Overclock Tuner to Manual
Then just make sure everything is at stock settings.
I bet what is happening is its unlocking your 4th core, and it is actually faulty so you cant use it.