What if remove 4 of 6 cards, i.e. leave only two cards? Do they both work properly? In my tests I used up to 4 cards, never tried >4.
I removed 4 cards,one plug in pcie 16x running @ 60k 4ch,another plug in pcie 1x @ 0k 4ch and GPU useage 0%.
Whatever, only the card plug in pcie 16x get work.
Well, now the only option is to reinstall Windows, it helps sometimes, see message above from "gilad215".