Ugh, the notorious 600B. I recently went on a tirade over this PSU being recommended by editors here at THW. Yes there, are probably worse PSUs out there (like RAIDMAX/Rocketfish/Diablotek), but do understand that these are cost cutting cheaply made PSUs. The OEM is HEC/Compucase, known for mass producing junk. On the internals, they use low end Chinese capacitors that start to cause problems over 30C when they fail to filter properly. When you put it in a gaming rig like yours, you are stressing that PSU beyond what it was made for, which is simple office desktops.
tl;dr Try a higher quality PSU like an XFX, Capstone or Supernova B2/G2 (not the NEX).
^^ If that doesn't fix it, then next we look at the motherboard.