Mitsubishi TV. My older model 35" tube mitsubishi tv becomes blank in the middle of the

Question

My older model 35" tube mitsubishi tv becomes blank in the middle of the screen after a few seconds almost like there is another screen popping up in the middle of the larger screen. There is about a 2" border around it where I can see the picture behind it and it is okay in that border. It goes away for a few seconds when I change channels but then comes right back. It does it on any input cable or DVD?