More concretely:

1) Best case: your power supply is wired in such a way that each GPU riser has it's cable to the power supply.

2) Check to see if whatever software method you are using that sets up TDP, cclock, mem clock, is working.

3) I flog this a lot, I have lost PC's because of poor wiring: http://www.homedepot.com/p/Commercial-Electric-Tools-Outlet-Tester-Green-MS112H/206029154