I have had this exact issue several times, and each time it was solved by tightening the coaxial connection at every connector...wall, cable box, TV, and including the connections outside (splitters, etc).
I'd try this first.
I'd try this first.
If you're not getting audible "hum" as well, then I doubt it's 60 cycle emf. It sounds (looks?) more like RFI which might be coming from a digital device.
Also, I understand it's not cool to put a CATV isolator (like the ones sold by Audio advisor) on your incoming cable if you're getting digital channels, because it screws them up.
Try unplugging all digital devices except your cable box (especially DACs and computers,) and see if that doesn't help.
In my experience this IS video noise from a ground loop. Tvad's solution may help if you are not getting a good ground because your connections are loose. Try an isolation transformer on the co-ax from your cable. If that works you know the problem; not sure about the solution if you have digital cable.
Swamp, how can we find out how isolate our system ground from the cable company without messing up digital channel reception? I could write to whoever makes the current crop of isolators and ask them (follow the money, which they're about to loose, right?) but I don't know who they are (I've always made my own ;~))
"It's always something . . . . " Gilda Radner
Try to determine the path that your cable takes (from where it enters the house to the outlet that you connect your TV to) look for the possibility that it travels with AC power lines. Mine enters the house next to the meter and comes up through the same holes as the romex that runs to the receptacles.
This usually indicates that the electrician used the lazy-man's way of running the cable...he ran it thru existing holes that were drilled for the romex.
If your house has this, then the next call is to the cable company to have them run a new line from their splitter to your TV set.
Save your money on CATV isolaters and call Cox.