Busterb,
Have you installed the latest drivers?
Are you connecting directly to the cable or dsl modem?
If you are, its more than likely an interfacing issue between you and the cable modem. I've seen perfectly good nic cards go south with cable or dsl modems that cut corners on following standards.
There's a reason why I like realtek 8139 pci network cards for any interfacing with cable or dsl modems when I have to. The card is cheap and works with everything you can throw at it.
Try the latest drivers. You really don't need to return the mb. Chances are its the other device
.
Mitch