Hey guys,
(let me 1st say sry for my bad english!)
I have the same issue: when I add 5th card my rig goes to boot loop.
And I did all that was suggested (even disabling 10 USB and 4 SATA connections of my mobo) except installing windows using a UEFI USB, I simply installed it by using DVD.

Anyway I overcame this shirty thing by not using the 16x PCIe slot on my mobo cause if any of my 4 GPUs is connected to 16x slot no matter which slot I use for the 5th card it goes to boot loop.

So right now I have 7 GPUs connected to mobo by skipping the 16x slot and everything is working perfectly. I just can't seem to get what the fork is wrong with my freaking 16x slot! 

BTW I tried Gen1-Gen2 and not Auto configuration but I don't think that is what making the problem.
Any Ideas
Thnx