If you're using a Motorola Surfboard modem, go to
http://192.168.100.1 in your web browser, go to the Configuration tab, and change the upstream channel to something between 1 and 5, but obviously not the number it is now.
I had the exact same problem as you just 3 days ago, changed the upstream channel from 5 to 3, saved it and rebooted the surfboard, and now it's fine.
Hope this helps
