Ok, so it's now at 40 hours uptime, and still no serial port crashes. I think the problem is truly fixed. Still not exactly sure what the cause was, however the fix came via a combination of enclosing the uno in a metal box attached to ground (EMI suppression) and changing the USB cable. I noted the old USB cable had an issue with its ground, the resistance was unstable. It varied from ~70ohms to over 300ohms. That could definitely cause problems. Swapping with a normal USB cable (0.2 ohm resistance in ground) seems to have fixed the problem. However, one of the first steps I tried before was swapping that cable, which didn't fix the problem at the time. So, I believe there were also some EMI issues which the metal box seems to have resolved nicely now. Hopefully this information is helpful for anyone else who runs into a similar situation.