i also got this issue,
im running 2x 1500 watt psu's on 4x Hd7990 Ati cards.
first time i booted the system it was running stable for two to three weeks everything worked fine.
and someday i came back cause my hashrate dropped to zero and checked the rig, cg miner reported temprature on gpu 1 511 degree and the rpm from gpu 3 & 4 was zero. so i checkt gpu 3 & 4 and the fans were running like they always do..

now everytime i boot my system up it runs stable for 10 - 30 mins and then the same thing happens gpu 1 511 degree
and fan speed of gpu 3 & 4 is zero, then windows freezes and then my rig shows a complete black screen and doesnt do anything anymore so i need to reboot the system and then everything works fine till a start cgminer and then after 10 - 30 mins the same thing happens.

does anybody know what this is?