I use motherboards 4 ea  for 3 month later. The PC's always ON and working everyday. 1 month ago, one by one PC is OFF itself and can't to power ON again, I try to remove power cord, leave for 45 minute, then PC can power ON again. But every 1-2 hours, the PC OFF itself again.   .
For information, temperature room keep at 20deg C. CPU and GPU monitoring at 40 - 59 deg C. only one GPU using, and other GPU just standby.
I was try to change Power Supply with Corsair HX1000, problem still exist.
Any one has a problem like me???
 
My PC Spec :
eVGA X79 Dark BIOS v2.11 
i7 4930K @3.4GHz HT on 1.2-1.3V
4x4GB Crusial micron DDR3-1600 CL11 @1.5V 
GPU 2ea GForce GTX 780Ti SLI system
sys @ silent custom loop in LL PC-D600B
Seasonic X 1050 80Plus Gold
Win 7 Pro

At this point you need to rule out as much as possible by stripping down the system to a basic computer. Just minimum hard drive, GPU and maybe just 2 sticks of memory and see if the problem is still there. 

Do you have any Win 7 utility programs that may be overriding the system and shutting it down, like System Mechanic for example?

Yes you can separate and run only one at a time.Just remove the jumper ribbon cable and 1 card. Once you are done troubleshooting you can install both and re-enable SLI. Just make sure to leave the single card in the top slot with the monitor cord attached.

Thanks for your attention MhzManiac .... I'll try today.....

Take a look at the logs and see if there any programs or errors associated when it happens.
 
To view the system log in Event Viewer
Click Start, point to Programs, point to Administrative Tools, and then click Event Viewer.In the console tree, click System Log.To sort the log alphabetically and quickly locate an entry for an Exchange service, in the details pane, click Source.Double-click a log entry to open an event's properties page. 

Sajin

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I just try to troubleshoot with minimum system, 1 HD OS, single GPU in the top slot, ..... Problem still exist, it's OFF itself after 30 minutes, and can't switch ON again.
Please .... any suggestion for me??? I have 3 PC same config., and same problem......

Now take the video card that is in the system out, and replace it with the other gpu and test again. If the problem occurs again the motherboard is most likely the problem. 
 
3 rigs with the exact same hardware having the exact same issue leads me to believe...
 
#1 You got a bad batch of hardware. (Motherboards or GPUs)
#2 Your home has faulty wiring that is causing the systems to shutoff.

woww..wooww..  Sajin about #2... your opinions... I operating 12 PC's with many products of mobo, but only mobo X79 was problem.
I will do your suggestion next step ... many tanks guaeess

Sounds like it's time to replace your x79 motherboards, or you could rma them if still under warranty.