First thing is take a look at these two bitmain help items:

PCBs: https://bitmain.zendesk.com/hc/en-us/articles/204875015-How-to-diagnose-S5-hashing-PCB
Hashing at 600: https://bitmain.zendesk.com/hc/en-us/articles/203559969-My-S5-hashes-at-about-600GH-S

Hopefully between the two that will help.  If not tell us more about power and PSU.