Since I don't have this TV, I am unsure what holding down number 5 will do. What you are seeing is noise from the lines normally not displayed between frames. There are scan lines that are not meant to be displayed which carry information about synch and even the closed captioning display. These look like noise when displayed and appear as a black line with moving white dots and bars inside it. This is due to the fact that your TV has very little "overscan." That is most TVs will have a few lines top and bottom which are cut off so that this noisy information is not displayed. It is not a problem with your display per se, nor is it necessarily a problem with DirecTV equipment. I also have DirecTV and occasionally see it on SD programming from a few of the OTA locals. Technically, the problem is with the transmission because these lines should never be visible.
There is probably a way to set overscan on your TV so that this disappears. In my case, I have set the overscan to only 2%, and it is no longer visible.