Pluging in the cards one by one in consecutive reboots worked, thank you for the tip (8 hours uptime so far) 
Try gen 2, i switched to Gen 2 too.
I have the same mb, I gotten 5 cards to work, with 4g decoding turned on, and all settings possible set at gen 1, when I plug in card 6 it shows bios splash 2 times then black screen.
Try gen 2, i switched to Gen 2 too.