Make sure you have the correct settings in Networking configured. Try to use an assigned IP address (instead of DHCP), provided there are no conflicts with other IPs on the same network.
Last but not least, make sure that switching the cable all the time with other devices have not physically damaged the ethernet connector on the PC. If so, you could try to replace it or get a network expansion card for it. Check the small metallic wires of the connector, see if they are not bent or broken.