Ethernet Port Problems

I have a weird little scenario and maybe someone here can help.

When I plug my ethernet cable into my imac, the network connection will be available for only a couple of seconds before the computer says that the internet is not available.

When I plug my ethernet into a wireless router, the network is fine. When I plug an ethernet cord from the wireless router to the imac, the connection is fine, even after I turn off airport so I know I'm using the direct connection.

How come my computer won't read the direct connection from the cable modem? It can't be the cables because I use the same cables for the router setup.

Thanks for all of your help!

Wait an hour. Cable company's systems are set up to give an address to the device connected to it and only that device for what is called a lease period. Usually 1 hour. After the lease is up you can plug in a different device and they will give an address to it and lock to that device .