Pro Micro QCM1250, freezes while trashing wired LAN

I put this into service about six weeks ago. It's been running fine except (and this is a big 'except') about once every 3-5 days the following happens:

• The system becomes completely unresponsive. The monitor is usually (maybe always, can't remember) off, and nothing I've done helps except for a hard power cycle. I've tried CTRL-ALT-DEL, ESC, disconnecting the network cable, shutdown keypresses (though I can't see what I'm pressing since the monitor is off) -- no dice.
  
  
• The processor must be very busy because the chassis and exhaust are quite warm
  
  
• The other wired ethernet devices on my LAN lose connectivity. Sometimes rebooting the router is necessary to recover network connectivity to other devices on the LAN. The router's wifi continues to work.

This freeze can happen whether the PC is awake or whether it's in modern standby in the middle of the night.

I've tried a bunch of things (none of which has helped) including:

• Running DISM and SFC to check Windows integrity
• Updating drivers and BIOS
• Disabling wake timers
• Running hardware diagnostics (all passed)
• Swapping out my router for a different router
• Disabling some services and scheduled tasks that I didn't think were necessary

The computer is only used for perhaps 3 hours a day, but still, it seems odd that the problems usually or always occur when there's no user present. For either a hardware or software problem, one might expect failures to be much more likely to surface when the machine is busy.

Any thoughts?