Go back to the basics. Clear CMOS or reset defaults. Do not try to unlock the hidden core. Now, using the X3 as and X3, try your hand at overclocking.
Step 1: Disable all power saving features (C1E, Cool n' Quiet, etc)
Step 2: Manually configure your RAM to the specs on the sticker affixed to the side of the RAM
Step 3: Raise your base clock by 5 MHz. Save and Exit. If Windows loads, go back into BIOS and raise the bclk by 5 MHz again. Repeat until Windows does not load.
Step 4: Now that Windows will not load, drop the bclk by 1 MHz until you can get back into Windows
Step 5: Download, install, and run Prime95. Run the Blend test for 3-4 hours, or as long as you can. Most of the pros on this forum run Prime95 for 24 hours before really declaring stability.
If you can get through this, your CPU overclocks just fine. On the other hand, if your attempts to OC were with the 4th core enabled, then chances are the 4th core is just defective.
Edit: revised the procedure for clarity.