I have appeared to have narrowed it down to the router. I turned off ‘Shortcut Forwarding Engine’ in DD WRT and I’ve been running successfully for a few hours now.
I tried hooking PC, Pi, and NAS to an unmanaged switch (then to the router) and it was a lot longer until the problem showed up. Whether or not that means anything, it gave me the idea to switch off the SFE in router.
Thanks so much for you guys looking into it for me, I appreciate it. Here’s hoping it’s all solved. Cheers.