X6 Randomly Rebooting - what to check in logs?

I have an X6 top-load (yeah, super old) that was still serving my purposes fine until it began randomly rebooting a couple of days ago.

I noticed RAID logs indicating what looked like a loss of power, followed by a power up and re-sync. This process seemed to repeat every 5 or 6 hours. After getting stuck in a perpetual file system check, I did a factory default restore and noticed (ugh) that it appears to be doing the ol' reboot again. I've hooked up another power supply and had the same issue happen. I'm currently running the memory test but I did a full download of logs and the Diagnostics.log indicated that memory passed. The system is on a UPS, so I don't think it's a case of a flaky outlet.

My question: what should I be looking for in the full logs to attempt to determine the cause of the random, mystery reboots? Any tips would be greatly appreciated, as I'd love to avoid having to put the old girl down.