DSL Connection Hijacked

I have a small network with a Netware 5 Server and 5 workstations. In the past 3-4 days I have noticed that at about 5:30 pm or so my internet connection goes down or is sporadic at best. I have called my ISP and they have tested my DSL router and they said that there doesn’t seem to be anything wrong. My ISP requested that a test be done on the line and that came back fine also.

Is there any tool that I can use to see if maybe
my DSL line is being used to transmit anything that I don’t know about? Could I have a virus?

Any help would be greatly appreciated.

Thanks