Ungrounded CATV coax connections can cause some rather bad speaker hum (at 60hz here in the US), and horizontal lines in your picture. Grounding your coax connection can help alleviate these problems, though it may not fully solve your issues. To demonstrate, here’s some backstory.
We have a older CRT TV in the garage that we use to play DDR/Stepmania, as well as have a TV going when working in the garage. I recently hooked an old computer up to the TV for a permanent Stepmania machine. Right after hooking this computer up, I started getting a really bad ground hum on the TV, which could be quickly fixed by installing a ground lift (Cheater Plug) on the computer power connection, but this is generally a bad idea (computers like being grounded). After doing some research, I found out that an ungrounded CATV connection can cause ground hum. Checking it out, I removed the CATV connection from the TV, and viola, the hum was gone. Okay… so, now to actually FIX the problem.