Usb modem interface is being dissabled

I am running windows 2003 server standard Edition.
it is used among other thing to router the internet to lan. A dsL modem is used with a usb interface. the problem is that the usb interface in the network connection window is being disabled.When you try to enable it, it will not. It behaves as though it as frozen up. I would normally restart the server. then when it boots up
the port is missing. so i unplug and replug in then it returns. But only a few minutes, then it trips out again. What do you think is the solution?