network cable unplugged

My computer randomly disconnects from the internet and a "network cable is unplugged" warning pops up when it does. I have checked all connections, disconnected and reconnected them, changed the ethernet cable and nothing changes. Even when I hooked up a different computer to the line I have the same problem. I have changed modems, changed routers, changed the phone line and the problem persists. What could be the problem?

 

Sounds like it could be your ethernet card.