5gpu 2psu scrypt mining rig shuts down 1hour later 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..

Regards,
My brothers machine was doing this also. We fixed it by pulling a card. The motherboard was pulling too much power under load.