JVC 27" CRT TV AV-27BP3 display shows single...

Question

I have a 27" JVC AV-27BP3 CRT TV and occasionally the picture becomes a single horizontal line at the center of the screen. If I bang the set, or cycle the power it off/on (after 30sec-1min), the full screen sometimes will show again. After fully cooling, it seems to show a full screen most of the time. Sound and other functions all work fine.The TV is 13-14 years old. Thanks for any suggestions.