How to Troubleshoot a PC that shuts off under high load

[Duox7142]

Hi all, I'm helping set up a Opnsense router for family, and used a refurbished HP 705 G4 with a dedicated Intel PRO/1000 PT Dual Port Server Adapter.
While hitting the system with maximum traffic (running a bufferbloat test or downloading at 100% of bandwidth), the PC will occasionally and suddenly reboot.
What is the best way to diagnose the issue? Is there a system log that could show errors prior to crash? My initial impressions of temperature seem fine, it happens in short manner and temperature never exceeds 80c which should be fine for a A6-9500 CPU. Power supply could be a culprit but a 65w CPU and an Intel NIC should not exceed the 180w 80+ platinum PSU installed. Though it could be a bad PSU, that would be the hardest to test. If there's something I could run to specifically test the error, I'd gladly try.
Thanks!

[johndchch]

first thing I'd try is a different known good NIC

Also the pro/1000s are pretty ancient - ideally you want something newish and with good driver support ( pair of i210/i219s or maybe an i350-t2/t4 or even an x540-t2/x550-t2)