7x 7970's, PCI-E 16x-connected cards not working

[-ck]

Try increasing your PCIE latency from 64 to 96 in the bios. Worked for me when I added my last 7970 when nothing else would.

[oldersol]

Had the same prob on a new GA-Z87x-OC board > mining on 2 cards worked fine > after adding 3de or 4th card WinOs did not find the card and the board went unstable during boot.
After setting the pcie or peg Gen config from auto to Gen1 in the bios > all went stable and solved the problem for me.