"Network cable unplugged"

Greetings and salutations.

Here's the problem. I'm using a (very retro) ZyXEL Prestige 630 ADSL modem (USB) for UTV's Clicksilver 3meg package. Since yesterday, every few minutes (the interval varies from a few seconds to a long while) Windows kindly informs me that a network cable for the connection has been unplugged, and the ADSL signal icon in the system tray turns blue as opposed to green, indicating a loss of signal. Of course, I can't use it until the signal magically comes back. By itself. For no reason.

One possibility is that my phone line is dodgy, however when I plug a phone into the line, I get a crystal-clear dialtone and I've had no previous problems with it since it was replaced all those years ago.

This has actually been happening on rare occasions ever since I signed up to their service two years ago. In the past when this has happened, sometimes I would not get a signal until I unplugged the USB cable and plugged it back in, or disabled and re-enabled the connection in Network Connections under LAN/Broadband connections.

Any idea what the problem is? Thanks.