CCTV Defective?

I have a Micromark CCTV (From Homebase a few years ago). The TV has horizontal dark lines which go downward on the screen. (Almost like an old fashioned TV line hold out of adjustment.) I thought the problem was the TV, but I fitted another one I know works on Digital/Video/DVD and had the same result. I have looked for poor connections on the wiring and checked the wires to the camera, but nothing obvious. I tried the Micromark helpline, but it no longer exists. Any suggestions what to try please? (Can't afford a new one yet!)