Old thread I now, but my recent experience may be of use to others.
My Ethernet by way of the docking station > router has been continually dropping out over the past few day. Falling back to WiFi. I am on Windows 10.
I tried various fixes;
ReDock SP3, Different LAN cable, reboot SP3, remove/uninstall Ethernet adapter from Device Manager, Checked windows update. All to no avail. Ethernet would return briefly then drop out. Unable to establish Internet connection, No IPv4 or IPv6 connection or IP. Interestingly my Fritzbox router could readily see the SP3 connected at 1Gbit/s and was displaying its mac address.
The fix was surprisingly simple.
Remove the power lead from the Docking Station for a couple of minutes having undocked the SP3.
Connected it all up again and good as gold ever since. I had overlooked the fact my Docking Station & Ethernet adapter therein had probably been powered continuously for a month or more. Cycling the power to it seems to have reset and fixed whatever was amiss. I gleaned this suggestion from a Google search and thought it worth mentioning here in case anyone else has the same issue.