Graphics Power Cycling Issue

I have an intermittent issue with my PC where sometimes, if I turn it on or reboot, nothing is displayed; no post, nothing. I haven't managed to find a common cause for this, though it happens less frequently when I boot to Windows, rather than Linux. There's no "Windows wasn't able to start" message when I button the PC and there's no startup sound so I assume that it's not actually loading the OS. I will need to verify this the next time it happens though, in case I'm not leaving it long enough.
The "fix" is to turn off the power to the PSU, wait 40(-ish) seconds then turn it back on again.

Any suggestions on how to fix or what the cause might be?

i7 5930K
Gigabyte UD5-Wifi with F10 BIOS (started happening before updating).
4x 4GB Corsair Vengeance DDR4 2666MHz
2x EVGA SC GTX 980
OCZ ZX 1250W

Do a CMOS reset or "load default settings" in the BIOS and boot with a one card at the time and put them through their paces with a test suite to verifiy if something's wrong. Also check if both of the PCI slots you're using are working correctly. Also check if the SLI bridge you're using is good and not defective.
What code returns the motherboard when refuses to post on the little debug display? That might lead you directly to the solution.


Replace the position of the cards ;)

Thanks for the suggestions. It took me a while to get any further with this.

While testing with booting to Windows I established that the BIOS splash screen doesn't show, nor does the OS loading screen but Windows does load.
I've yet to try swapping round the cards for good measure due to lack of time working on my PC this week but it's next on the list.