exactly, athlon x3 use propus (x4) die w/ a core disabled, so there's a chance to get d disabled core unlocked. athlon x2 is on diff architecture where there are only 2 cores on d die, there's nothing more to be unlcoked. 
try to force tri-core by selectin either one of 4 cores to be disabled in bios, if still not stable then it's obvious that d locked core(s) is/are not stable, eventhou they can be unlocked. 

success unlocker here...X2 555-->X4 B55..currently usin GIGABYTE MA785GT-UD3H board...overclocked to 3.82GhZ cooled by Zalman CNPS9900NT

my phenom II x2 555BE unlock result. So far stable. 

There is a reason why the core is locked in the first place
If you already unlocked all 4 cores and it is unstable even at stock voltage, means he unlocked cores are bad
Try rebooting with 3 cores and see? If not possible, you have to drop down back to 2 cores (but remain the unlocked cache if there is )

based on my experience also,740 is not a good chipset for unlocking cores..will do best on 770 or 785..
but u can update the bios,u r using MSI?..better refer to MSIHQ-USB flasher guide for BIOS updating coz most of BIOS Rom they provided will make ur UPDATE ROM MISmatch and if force to proceed,u can say hello to new MOBO already..