Zenith console tv model sentry 2 27 inch

Question

the picture will start normal (full screen) and then shrink down to a small band about 1/3 the size (in height). the picture is actually sqished at this point. The screen still has good clarity and color.
If you tap the curcuit board (lightly) or even the side of the tv ( a little harder) the picture will come back for a few seconds then shrink back down. can you help me with a possible solution to my problem?
Thank you,
WJ Carlilse