Hello.When I try to unlock cores, Windows 7 sometimes boots, but sometimes it doesn't. When it boots its perfectly stable. Should I change anything more in BIOS/Windows ? and what is the maximum stable voltage for the 555?
More aboutphenom unlocking problem
The X2's have locked cores for a reason. Sometimes they are stable, sometimes they are faulty.
Try unlocking only one of the two locked cores to see if you can at least get a stable X3. If not, you are out of luck.
Please visit an overclocking/unlocking guide. hopefully 1 written for your specific motherboard.