It is possible that the ethernet interface is taking errors and going into a shutdown condition. You need to check the configuration of the LAN switch port where this router is connected. Make sure speed & duplex match. Turn on debug logging to get more details around the event. Please post more information when it becomes available.
Also, in addition to the duplex and speed of the Eth0 interface you might check the other interface. While it’s down try pinging the two IP addresses of the router. If you can ping them both then atleast the router is routing traffic and you’ll need to look at the external connection.
It is possible that there is a speed/duplex problem on the internet side of the router, check with your ISP to see what they need the interface configured for because sometimes AUTO configuration doesn’t work properly between different device types.