Sounds as though your running too much from your USB port. Broadband modems tend to draw the full 500ma current and using other USB hardware at the same time may cause drop-out of the connection.
This can be solved very easy by purchasing a self powered USB hub, about £20.00 and connecting your broadband through this. it will give you four ports in total and you will be able to use the other three at the same time as your B/B. You can also carry on using your existing USB connectors for other items. j.