Monitoring app?

I`ve recently had an Atlantic Webstar cable modem installed, no ic and
no etheernet card. The modem is connected to my computer via a usb
cable.
The problem i`m having is that at varying times all net activity ceases.
This can happen after less than a minute but usually after around 15-30
minutes.
Wondered whether there was a monitoring app that might help me ascertain
what was actually going on when these net freezes occurred?

Paul