I have a NAS that had a similar problem, every week or two it would stop responding. From the logs it was clear that the machine was running fine after going non-responsive, but either the NIC or something else network related was crashing. However, the logs weren’t capturing the exact issue. I wrote a script to ping Google every 10 minutes, write a bunch of debug data to disc if it failed, and then reboot.

And it has never happened in the year since. I have no idea what the root cause was, but sending those pings out seems to have fixed it.