Hmm. I had a 555 BE on an MSI 785GTM-E45, and every time I enabled the "Core Unlocker" feature, whether or not I enabled ACC, it simply wouldn't boot.X2 555 unlocked to X4 B55 @ 3.8GHz 🙂
Oh, and its from batch 1009CPMW and on an MSI 785GTM-E45
Does your MSI board have a setting for "EC Firmware"? I couldn't find one on mine.
Also, did you update the BIOS? There were no updated BIOSes listed on their web site, but only links to their "Live Update", both options which didn't work.
I had an X3 710 that unlocked. I could get it to ~3.2 stable.