I had stability issues with my system, so started digging into stress testing of various components. It turns out that it consistently fails prime95 (mprime version - for linux) even without any overclocking. Tried to bump up vcore voltage without any success - still fails. And it fails only after a few minutes of running the torture test. memtest86+ completes successfully, so doesn't look like there are any obvious memory issues.
Any ideas besides opening an RMA case with AMD and getting them send me a new CPU?
Thanks for ideas. After lots of frustration the issue was finally resolved. It turned out to be RAM, even though it passed memtest. Replaced OCZ sticks with Crucial with the same timings and everything is OK now. It did fail large FFT test and was stable on small FFTs.