Horizontal scrolling lines!?

You have the classic symptom of a ground loop problem. Something is not grounded to same reference ground as the rest of the A/V equipment. The odds are that your co-axial RF cable line is not properly grounded because this appears to be the most common cause of ground loops with new HD TVs hooked up too cable TV systems. Do the horizontal shaded lines go away when you disconnect the co-axial cable coming in from the cable system? I had the same problem with Comcast a few years back and solved the problem on an interim basis by running the cable line through the RF connectors on my surge protector. In this case, it solved the problem by grounding the cable line to the surge protector ground.

If the problem turns out to be the co-axial cable line, you could call Comcast and ask for a service call to fix it. Good luck in getting the CSR to understand what you need to have fixed. Or if you have access to the cable box coming in to your house, open it and fix it yourself if you know what you are doing.