New build black screens and blue screens

[f2bacon]

I wanna try and long-story-short but it's a bit of a long story no matter how I tell it, so here goes nothing.

I've done a couple builds for friends, all very similar to my own build (rx580, asrock b450, ryzen 5 1600, and so on)
but my friend wanted to do a couple steps up with a more modern build (sapphire rx5700xt, asus x570, ryzen 5 3600x, evga 700w g3 psu, 16gb 3000mhz corsair lpx raim, 500gb 970evo nvme ssd)
To start it off, it wouldn't power correctly. The motherboard would light up for a second when the power button is pushed, but would immediately turn off. I took it apart for him and put it back together and even at best, the motherboard would stay lit up but wouldn't output display (gpu or hdmie on the mobo).
I took a wild guess and said maybe a faulty power supply. So we ran to bestbuy and price matched a corsair 600w. Now the mobo clearly lights up, but still won't display on any monitor, from any port. I figured it was that the mobo needed updating so I threw in my old ryzen 5 1600, no luck.
I start going for broke and say yaknow what lemme pick up an asrock b450 just because I know it's worked every time. Turns out everything works fine. It boots like a champ, we even carried over his old hdd so he didn't need to install windows after about 7 hours of troubleshooting.
He returns the old psu and mobo and ended up saving over $200. He was great for a while and then started having black screens and blue screens. He'd be fine for a couple hours and then it would go down. He said he couldn't pinpoint what's causing it.
At this point I assumed all the hardware is good, and it might be his old windows install and some driver issues or something. I figure we should do a fresh windows install on the nvme regardless, so we do that. The windows install is great, works fast, he loves it. But the blue screens and black screens don't stop. 
I'm now completely lost. I almost wanna try swapping out the 5700xt for a rx580 just cause every time I stray from the build I know well I get punished. All motherboards were on the most recent bios.
I looked over the forum to check if this was a known issue but nothing popped out. Am I missing something? Is this an easy solution? Thanks for absolutely any help!!!