I was suspected of mobo too, gonna send it back and see how it goes.
So the problem was not mobo either, and it was cards from different manufacturing patch.
What I got from the seller was 2 big boxes with 10 cards in each + 4 cards, seems like one of the big boxes have different memory type.
So I got 14 good ones and 10 not so good ones ... lol
So now the question is which combo is better ?
6+6 in each rig
or go with one fully good and the other 4 good and 8 not so good ?
argh... but you stated before that all cards had same memory... I assumed that you tested each and every one with gpu-z under windows... that's what I did with mine...
Anyway, at least you know where the problem lies now... if you can't return the cards with "bad" memory I suppose it is best to make one rig with all "good" cards and the other one mixed