Zenith monitor in an arcade game
Check to see if you can find horizontal and vertical sync controls. These would typically be small knobs or screwdriver adjustments on the edge of a circuit board or at the back of the monitor chassis. Try adjusting the vertical sync....rotate the knob back and forth two or three times to "clean" it...then adjust to see if you can get the picture to settle down.
Aging electrolytic capacitors are the most common cause of symptoms like this. You'll need the assistance of an experienced electronics technician to find the exact source of the problem.