Can you get us a photo of your rig? In most cases, this is due to the position of the graphic cards.
They are all hanging freely in open air. Several inches between each of the 4 cards.
I have 3x7850 +1x7970 working. But suspect I can get more kh.
Motherboard is "MSI Z77A-GD65 Gaming" 16GB RAM, Ivy Bridge i3 3225.
These are all off 1x to 16x adapters and strung up for cooling.
Temps are 63 82 56 61. The 82 is one of the 7850s, not the 7970!
I tried re-seating the heatsink. Seems like just a hot GPU chip.
Desktop runs off the 7850 in the 1x slot closest to the CPU.
Drawing 680w. ltc mining about 362 kh on each 7850. But only 478 off the 7970.