New build keeps crashing

I recently build upgraded my computer with new mobo, CPU and ram. I kept the same PSU, GPU (GTX 670), case and drives. The new parts are i5 6600k, z170x gaming 5, and 16gb ddr 4 corsair vengeance. Os is installed on an older SSD (512 crucial mx100).

When I first built it, just to see if it would work, I booted up my previous install of Windows 8.1. It worked fine. I installed the proper drivers from the CD, and the device manager was happy. Until it crashed. It restarted once or twice a day, as if the reset button had been pressed. No dump files or logs beyond "unexpected shutdown".

I figured it was related to overlapping drivers or something like that, so I formatted and did a fresh install of Windows 10. Still crashing, now a couple times a day. I updated the bios from 4 to 5 (last October to this April) and that seemed to reduce it to once every day or so but still happening.

Now when it happens though, side the bios update, most of the time it reboots, it goes to "insert boot media" ad if it doesn't detect my SSD. Reset/ C-A-D does nothing, but a hard shutdown and it boots up just fine. The crashes never seem to be when I'm doing anything, in fact I don't even see them, just come back to it, so it's not a stress thing, and is not a sleep issue cause it's disabled. It's also not a temperature thing as I have a new water cooler and the CPU is downright frigid.

I know that my SSD is technically not supported by this motherboard as per both the Gigabyte site and the crucial site.

What does this actually mean and could that possibly explain these crashes? For the time being Ive just plugged in my old hard drive that I'd used to image the ssd when I got it, just to see if it still crashes, and have the SSD completely disconnected.

If it's not the SSD, I'm thinking it's probably the motherboard.

Thoughts?

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