i had some what the some problem with my first rig evga x58 ftw3, i had to get 16x riser on the 3 pci e and one power 4 pin on my four card and i wouldnt get 100% on my 3rd and 4th card, so i ratter keep my rigs at 3 cards.
you could always get your self some use parts on ebay to run an extra card or 2 , did it my self got my self a dual pci e board for 50 bucks on ebay
and i was able to run 3 cards on it.
well that would really be the last thing I would do, since I know people are successfully using 5 GPUs on an Asrock 970 Ex4, but I'm starting considering it at least temporary, just the time it takes to solve this matter as I don't like having an extra GPU sitting there idling.
Hi suzanneg,
This is exactly where I got my info about building a rig! Best tutorial I could find so far!

The difference is that I intend to use 4 GPUs with 4 powered PCIe 1x to 16x risers.
Hi cryptoprime,
Yes I'm considering doing this, but I can't find info from owners of the same mobo.
Do you know if I can safely short a PCI x16 slot? I found infos about shorting a 1x slot, but nothing about the 16x slots.
Also I read stories from people frying their mobo shortly after shorting their PCIe ports... simply horrifying..

What psu are you using?
Hi justkid,
I have two Akasa Cobra 850w running on the same rig, which are intended to power two GPUs each. With a total of 1700w, I think that might not be the issue.
Maybe one of powered riser's is damaged?
I dont know for sure, but a PCI x1 slot just got about 500Mb/s in each direction and my solo mining gpu produces a heavy amount of traffic. when start copying files between 2 harddrive's while mining, i just got half the i normally got.
So maybe a PCI 2.0 1x slot hasn't enough bandwith to fuel a HD7970/R280X

Well I have 5 PCI risers, and I tried to use only one at a time while the three other GPUs are connected directly to the mobo. The ones on the mobo are detected, but not the one on the riser. I tried each riser, unsuccessfully.
I even tried to short the presence pins, but no change...