What device is connected to the RUT240 WAN port?
Do you know its speed/duplex settings? Maybe there is some mismatch, as I see the link negotiated to 10Mbps?
Could you try a different ethernet cable?
Could you connect WAN port to a different device, serving as a DHCP server, or configure it as LAN port (instructions here) and connect it to your PC to check, whether the same port behavior reoccurs when connected to different devices? Repeated behavior could be indicative of router's hardware issues.
One more way to try to capture more data is to install TCPdump package from router's WebUI Services -> Package manager page and enable TCP dump monitoring on eth1 interface. Keep the device running for a while, if possible, connect WAN port to more than one device, download TCP dump file afterwards from the same page and upload it by editing your question, or in a private message.