Ah, there's the rub. Is it the CPU or is it the board?
To be totally honest it's more likely to be the CPU than the motherboard and I have come across several instances where an unlocked CPU exhibits this type of behaviour. Just one of the reasons the cores are locked during QA testing by AMD.
You might not be able to get both of the "extra" cores to unlock successfully. Have you tried just unlocking one and seeing what happens. Of course, you might have to play around a bit to find out which of the "locked" cores is the most reliable but that's part of the fun. And, of course, an X3 is still better than an X2.
Another option would be to try a slight increase in power to the unlocked CPU as this can often help improve stability. But be careful if trying this, as extra voltage through the CPU means extra heat and if you are using the stock cooler you won't have too much headroom in that area to play with.
The only real problem with the motherboard would be the BIOS and, if you are not running the latest version it might be worthwhile updating to that. But again, I must add a note of caution, flashing BIOS could make things worse, so be prepared to go back to your current version if you need to.
