If your broadband is connected via USB. That's not suggested by most.
The disconnect can be proper since you may have set USB to be power managed. Look at that dialog box. Here's a picture.
See the Allow computer to turn off power? You decide what to do here.
Just recently on my Core2 Duo Dell, I started to get the "usb connect/disconnect chime" at random times throughout the day. Sometimes our broadband cable loses its signal. Could this be the cause? Connected USB devices are: mouse, keyboard, printer and a rarely used USB cable for an mp3 player. All devices work ok. Ideas?