Hey, I got the same thing when a set up my rig.

I tried to make a rig with 3 7950 with a 760 watts 80+gold psu. The 3rd card was like your one, only 80kh. I switched many times, but the thrid pci-e slot refuses to make more khs.
And when I used only two card, even on the 3rd pci-e slot, everything was right.

So it ends up to the conclusion : my psu wasn't enough to supply 3 card, even with a gold psu. I would say that your psu is kinda broken, since 850 w is clearly enough to run 2 cards. Is that a 80+ psu ? Remember that 80+ means 80% of power delivered, so 850*80% = 680 watts