TV problem

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Does anyone know what could cause a TV (cathode type) to get a black bar at the top, with a thin light strip along under the black bar. When I turn on the TV it is several inches thick and then it gets thinner. But sometimes it is bigger when I am watching TV too. Other times it is barely noticable, but either way it is there everyday. I tried unplugging it for awhile, but it is still there. My TV is only a few years old. Does anyone know what could be wrong?