Nothing obvious in the logs, but from your description I would suspect a LAN cable (or port) is dodgy. A 1Gbps connection requires 4 twisted pairs in the to be functional while 100Mbs ( ~ 10MBps) only requires 2 twisted pairs to be operational. With a dodgy cable/port it is not unknown for the LAN connection to silently degrade from 1Gbps to 100Mbps or lower. Because the LAN keeps working a cable/port issue is often not diagnosed.