Specs:
i7 6700k 4.7ghz
EVGA FTW Gtx 1080 SLI, both oced
32gb Ram
gigabyte z170 gaming 7 motherboard
Dual 4k monitor, using nvidia surround 7680x2160, gaming with 4k res
PROBLEM: PC freezes constantly while gaming, but only 4k though, overwatch league(1920x1080) works fine, witcher 3, gta5 etc with 4k res cant last longer than 10 minutes, computer just freezes completely and restart itself or i have to restart manually, I think the driver is fine, also reinstalled win10 a few times, nothing is fixed. The only possibility left in my mind is the card themselves, or maybe it isn't, what other parts could it be that might cause this type of problems, and how can i test which card is causing the issue? should i just run single card for both cards? and also i bought them in August from the local retail store Canada computers, but the warranty is with EVGA only. Does overclocking gpu void warranty? Please help :( i really don't want to go thru the RMA, its so frustrating. Thank you all in advance! 

Feel free to email me any advice, ericzou@live.cn
Thanks again!

Question: Does overclocking gpu void warranty?
 
Answer: Overclocking our products does not void the warranty as long as there is no physical damage to the product or missing components. However EVGA Support will not be able to assist you in overclocking the product.
Source

Hello,
 
Can you try each of your 1080's by themselves in your system and see if one of the cards causes system freezing?
 
If you do this and you have the same issue on both cards it is extremely unlikely to be a graphics hardware issue.
 
Please let us know.

 
Moving to the 10 Series section.

#1 Set your cpu, gpu & ram back to default clock speeds.
#2 Retest system.
#3 Let us know if you're still having a problem.

thank you for the reply!

Yes, i tried using just 1 card for both cards and ran 4k games witcher3 and gta, both cards was stable and worked fine, they didnt freeze the computer, but the game itself crashed a few times with a message "witcher 3 has stopped working". It was around 45-52 fps running 4k witcher 3 for just one card, 60-70 for two cards, is the fps normal in this case, shouldn't it be higher since i oced? but getting same fps compare to founders edition sli not oced. Thank you for the reply!

I tried that as well, didnt work. :(
Thank you for the reply!

KugeIfisch
I tried that as well, didnt work. :(
Thank you for the reply!

It would help if you would quote who you're replying to so we know what you tried. What power supply are you using and are you using a UPS?