Strange problem with overclocking my Opteron 165

I've had my Opteron 165 for about 3 weeks now and so far i've got it up to 2.6GHz at 1.4 volts. I was happy with that for a while but then I wanted to see how much higher it could go. So I tried 2.7GHz but core 0 seems to be a lot weaker than core 1 because it couldn't do 2.7GHz even after increasing the vcore to 1.55 volts. Core 1 had no problems doing 2.7 or 2.8GHz. So I did several hours of burn in on core 0 and managed to get it stable at 2.7GHz at 1.52 volts. I wanted to get it stable at a lower vcore of 1.50 volts so I did more burning in. It seems like I was making progress with each burn-in session because it would Prime longer and it was Prime stable for about 9 hours last time. Now the strange problem is I can't even get it to Prime for more than 30 min at 1.50 or 1.52 volts when before it was stable at this setting. Is there an explanation as to why this is happening? I don't understand how it can lose stability.