Probably just aspect ratio settings. If it happens mostly on your local channels and looks a bit like static, then I am almost 100% sure that's what it is.
Try setting your TV to 16:9. Some channels actually have information around the outside of them, which you wouldn't normally see. And some channels have little white static lines across the top. Haha. Others have just black.
Without seeing your equipment it's difficult to say, since some cable boxes, AV receivers, and TVs will all have aspect ratio settings. But for the most part, everything should be on 16:9. I think Panasonic might call it "full".
If this doesn't help, please take a photo and post it.