What is the exact model number of the TV you are working with? It will say what it is on a sticker on the back. This can help give some idea of what may be going on.
Typically, as older panels fail, it is because of the connection between the main board and the LCD array on the panel itself. It could just require taking things apart and reseating the connection, but beyond that, it could be an actual failure of that section of pixels, which would require the entire panel to be replaced.