[MtnDave]

I had a Comcast cable modem for several years.
Assuming it is not a line level problem (which you should have checked), it may be due to your dynamic IP address timing out. The next time it happens, do an ipconfig/release, ipconfig/renew. If you are using a router, you will need to login to the router and do it from there. If the modem is directly connected to your computer, use a DOS window.