VaiZore

Hi all. I consider this resolved now. Thank you to all who offered assistance.
 
The solution - I moved the equipment into a new case... (Fractal Define 7). 100% uptime since the change (weeks, now).
 
Since I had already ruled out the power & reset switches in the old case, I have no idea what the problem could have been. The metal panel in the old case (also a Fractal, a much older one) that supported the motherboard might have been slightly bent outwards (convex), but it didn't look like it was far enough out for the metal to make contact with any components or tracing on the back of the motherboard, or any of the 'keep out' zones. Plus, I used all of the correct standoffs for the form factor, so it's unlikely to have been a short-circuit. Maybe there was some other defect that wasn't obvious, poorly shielded header cables, or some improper grounding or something... I'm not an electrical engineer, so take these guesses with a grain of salt!
 
Either way, it wasn't the fault of UNRAID, which I can now enjoy uninterrupted. I have since bought a second license for a backup server, which is also rock-solid. I'm very much enjoying the product again.

Thanks to @ChatNoir and @JonathanM and all of the members who contributed their assistance.