Zenith 1995 21 inch floor model - picture shrinking

[misterdd]

The model number is SR2504BW, it's a floor model 1995 21 inch Zenith. The picture shrinks to about 2 inches high horizontally across the whole screen. It doesn't do it all the time, but probably at least 10 times a week. The 2 inch viewable screen is about halfway up the screen, it will sometimes stay that way for only a couple of seconds or it may stay that way for 5 to 10 minutes. If anyone has any idea on what the problem might be and how to fix it, I would be much appreciative.