Network Cable Not Connected

I get the message on my desktop computer 'network cable not connected', while it's connected. The service prividers have a connection to the modem and I have tested the modem elsewhere and it works fine, so modem is not the problem. I thought the network cable was the problem, bought a new one, worked for 5 minutes then the message appeared, lost the internet connection, the problem persist, cable not the problem. Tried inactivating, then activating the connection to the local network, the problem persists. Tried sytem restore, still not working, disabling the firewall if it blocks the connection, still not working. Could the networks card be the problem? I updated drivers to the network card, didn't work, bought a new card, the problem persist, then finaly got a tip from someone to pull out the electric cable from the computer, then put it back again, the problem was solved. I tried this, but still have the problem.