I'm having one heck of a fun time trying to get more than 3 cards running on my various mining rigs. I'm using 3 Biostar A870 boards with both powered and unpowered PCI-E x1 and x16 extender cables.
2 boards are populated with 4 reference 6970s each, and are powered by Kingwin 1000 watt 80 plus gold PSUs. Running up to 3 cards works w/o issue, but using 4 sends my hashrate through the floor. MSI Afterburner also stops working.
The third rig uses 4 1 GB 6950s, and an NZXT Hale 850w PSU. This rig won't even detect 4 cards, but like the first, using 3 is fine.
What's happening, and what can I do to fix it?