So is a corsair cx750m not beefy enough for these? I have two S5 running on one corsair cx750m each (which only have two PCIe cables each that split to provide two plugs each for a total of four). I did have one unit shut off randomly after a couple hours of mining yesterday but I didn't pay attention to whether the PSU had shut off or just the S5. The S5 was power off though, no lights, no fan, nothing. Turning the PSU off and back on brought it back up and it has been running since. The other unit has been running the whole time.
The PSU is powerful enough, but you need to make sure the cables are not heating up. I had it running in a fairly cool environment (~20C) without issues. A PSU with 4 separate PCI-E cables would be safer.