OP, do you have corsair PSU HX1200?  Flip the damn switch from multiple rails to single rails as a test.  See how the rails work?  This particular bad ass PSU can switch between the 2.  I would choose multiple rails with proper loading.  As a test flip the switch if you have this PSU to prove the wiring load is unbalanced on multiple rails.  PS it come default to multiple

Corsair HX1200 DC Output Ratings in Single +12V Rail Mode
DC output   +3.3V   +5V      +12V   -12V   +5VSB
Max Load   30A            30A.  100A   0.8A   3.5A
Maximum Combined Wattage   150W   1200W   9.6W   17.5W
Total Power: 1200W

Corsair HX1200 DC Output Ratings in Multiple +12V Rail Mode
DC Output   +3.3V   +5V   +12V1   +12V2   +12V3   +12V4   +12V5   +12V6   +12V7   +12V8   -12V   +5VSB
Max Load   30A         30A   40A            40A   40A.     40A    40A    40A    40A    40A    0.8A   3A
Maximum Combined   150W   1200W   9.6W   15W
Total Power   1200W