Thanks for the replies. I read them when posted but I'm still working on resolving this issue and trying things.
I've tried:
-jiggling all the cords
-tapping all over the case, by the power button, by the motherboard rear IO buttons
-looked all through case for loose connections, misaligned parts, loose screws, etc.
-gently shaking system (only an SSD so no moving parts to get damaged from gentle shake)
It won't reboot during any of the above tests but I got up from my desk and bumped the desk fairly hard today and my computer rebooted.
It's weirdly random. It's not heat related as I can game for hours and it doesn't effect this issue. Temps are all great for a case this size, too.
Motherboard doesn't appear to be flexing at all. All screws are in tight. Only thing that's flexing is my SSD. The SSD heatsink on my motherboard is janky and causes the SSD to bow a bit no matter what I do. I don't think this is the cause of my reboots but I'll investigate this further. It's a double sided, two port card for the SSD's and I've tried both sides and the same thing happens. Performance wise I've extensively tested and the SSD performs normally.
Motherboard is an ASUS X570 Crosshair VIII Impact.