could be a ground loop issue? Is your incoming Cable isolated by a transformer? Many times attaching a house ground to the coax's sleeve will clean up noise issues, but the usual complaint symptom there is hum in the audio. However I'd first ensure that you are indeed isolated. Many archived threads on this: please try the search engine or just go into the older threads & look. They have suggestions on particular isolators & their prices posted there.
Diagonal Noise lines
I am having a problem with my video in my home theater.. There are diagonal noise lines rolling through the picture in only some of the channels. For troubleshooting, I have viewed other TVs on the house on the same main cable TV feed, and the noise is not there. This leads me to believe the cable signal is OK. For troubleshooting, I have moved one of the good TV sets with a known good video input cable into the room, and the problem still exists. I have also ran changed the connections on the splitter and the problem still exists with this one location. The only thing that I have not done is run an extension cord from one of the good rooms to troubleshoot the power. My power is going through a Newcastle power conditioner, but I have tried other power conditioners in the same room with no change. All of my video cable have been swapped one by one with the "good" system with no problem. I am on a concrete slab with steel siding. Any ideas??? I am pulling my hair out...