Hi,
I got the same issue with the new risers i got,  system running fine with 3 x 280x cards, but when i connect it via risers it works for few mins then freeze and 1/2 out of 3 cards shows temp of 511.

My config is :

PSU : Zalman 1250W platinum
GPU : 3x 280x dual-x sapphire
Mobo : GA-Z87X-UD5H

I did try switching mobo and psu from another rig and problem continues, i took 3 other new risers and problem continues...

Did you find solution to this weird problem ?