This is a build from June.
Gigabyte G1 GTX 1080
Gigabyte Z170X-UD5 Motherboard
EVGA Supernova 650W P2 PSU (80+ Platinum)
The build was rock solid, and I've had many extended gaming sessions on it. Probably just in the last week, I started getting hard shutdowns. I've been playing Titanfall 2 lately, and I'll go a few games before it shuts completely down, waits a few seconds, then restarts by itself. First thing I did was replace the power strip, though I didn't think that was a problem since the monitors and speakers do not shut off, and it made no difference. Then I went through and reseated all the atx cables (which I also was not hopeful of since I'd expect it to consistently die), no dice.
Tonight I did a few runs of Prime 95 + Furmark to draw lots of power and I've consistently been able to reproduce the shutdowns. The last one was at about 8 minutes of running after the reboot.
Normally I run in the "Eco" mode so the fan doesn't come on most of the time. For the sake of testing I tried turning it off so it always has the fan running; in the one test I did, it hit about 25 minutes of running successfully, before I switched it back off and let it go for a bit until it exhibited the shutdown. I'm about to do another run with the fan running and let it sit much longer, but I wanted to drop this in here first.
Might I be missing something? Is there anything else I can try before calling it the PSU? Otherwise I'll be submitting an RMA
EDIT: After the first reply I realized I left out two important bits of information: CPU and GPU temps. GPU holds fine at around 65C and the CPU maxes out at 80C. This is with aggressive fan profiles so the CPU is sitting at 100% fan speed and it's seemingly stable. I could probably leave Prime95 out since the CPU load ingame is relatively light, i.e. I may try reproducing with just furmark.
EDIT2: Last update in post #14. Turns out it was the PSU.