I built the following rig:
5x SAPPHIRE TRI-X R9 280X 3GB GDDR5 OC (UEFI) 1x AeroCool Templarius Imperator 1150W 80 Plus Gold 1x HighPower Tech 1200w Gold Plus (HP-1200-G14C-GOLD) 1x Gigabyte GA-990FXA-UD3 motherboard 1x W.D Caviar Green 500GB SATA III HDD 1x Kingston KHX1600C9D3B1/4G 2x Unpowered risers 1x 16x 3x Powered risers 1x 16x
I have the templarius PSU powering the motherboard, the HDD and 2x GPUs with unpowered risers. The other 3 gpus are powered by the 1200w PSU and connected to the powered usb risers.
I Installed Windows 8.1 and plugged one gpu. It automatically installed the Radeon HD 7990 driver. Then connected the second gpu and followed the same process with the other gpus.
I then executed cgminer and everything looks stable at 740Khs each.
However 1h later the rig shuts down. It always shuts down aprox. 1h later..