So, I'll just lay it all out:
Panasonic Veira 50 inch Plasma
Denon AVR 3805 Receiver
Sony DVD player
Verizon Set Top Box
Verizon Modem/Wireless Router
Turntable plugged into receiver power outlet
Very old house
Everything is plugged into one surge protector, on one outlet.
Upon installing the Verizon Cable TV, I noticed extremely faint horizontal bars. They only occured when the STB was connected and powered off. When you turned the STB on, they were gone completely (even if the image was very dark). So, I was convinced it was fine. At this time, the cable box was connected via component directly into the TV. Audio would output from the TV into the receiver via optical. Everything was fine.
Being as I have two other video outputting devices (the DVD and Wii), I decided to run them all through the receiver. Now, I get stronger horizontal bars that don't go away from video that goes through the receiver.
Cable TV: very strong horizontal bars
Wii: weaker horizontal bars
HDMI from computer to TV: no bars
TV apps (netflix): no bars
I tried turning the cable box off (and unplugging it), and the Wii still emits bars, so that might rule out a COAX grounding issue?
One other thing I tried was running the cable box to a different power outlet, and another time running the receiver from a different power outlet. This didn't fix it.
Any suggestions would be greatly appreciated! (please let me know if there's a specific product you use!)