Ok so I got my z87 stinger about a year ago because I fried my asus Maximus impact and 4770k in a storm, and I saw once before about 6 months ago that it shut down like this after sitting idle cause I feel asleep at my desk, but thought it was a bad windows install cause I couldn't install any programs. Did a clean install and never noticed it again till now. I recently purchased a nvidia shield TV to replace the Xbox 360 I had in my living room because it died, I saw the the nvidia game stream and thought well if I can play my games in the living room then move to my desk when my wife wants to watch tv that's a great buy. The issue is when I leave the PC turned on not being used directly for a bit it randomly reboots a lot of the time from what I have seen is I start game stream and then the system reboots about 10-15 mins into playing a game. Now I turned off screen saver and sleep mode in the desktop and the hibernation function in the bios but it still does it. Any idea of what my issue is maybe it's my hardware. I currently do not run any over clocks.

Current Setup:
Evga z87 stinger
Intel core i7 4771
Asus strix gtx 970
Corsair vengeance 16gig
Corsair h100i with 4 fans in push pull for top exhaust
Corsair ax860i
Samsung 840 evo 240 gig boot
Samsung 840 evo 500 gig storage
2 140 intake fans both front and rear
Inside a bitfenix prodigy case