^^^ If I had to guess (I'm guessing) your cards have crappy Voltage Regulator Modules, I'm guessing you bought the only cards available and they were still in stock because they suck.
I just got stuck with asus 580s elpida, same issue... i can pack 19 msi cards in one rig if I can find stock...
I got the cards a while ago, and I got them directly from the official distributor in my country where they can only sell at the original retail price.
I dont think any sapphire nitro+ limited edition are bad, they are all ASIC 80%+, samsung memory, and probably the best performance for mining.
The thing is, I tried to split the rig into 2x 6 cards each, and they work perfectly fine (the same cards I used in a 12 gpu rig),, but putting them into 1 mobo (tried ASUS B250 mining expert, Biostar TB250 btc pro, and Asrock h110 pro btc+), some cards crash, so Im guessing either claymore doesnt do well with 12 gpu rigs, or there is something Im missing.
I got plenty of virtual memory.
tried higher end processors and more ram
lowered core n memory
tried stock bios with stock core/memory and voltage
sgminer-gm works with 12 cards and all is fine, but it hashes lower than claymore for me (900hs for sgminer, 1050hs for claymore).