This would most likely indicate a dry solder joint on the chassis. To isolate the chassis, have you got another gameboard to try ?
I would not attempt to repair the chassis unless you have experience and an understanding of the high voltages that are present.
What you can try is adjusting the monitor (via the trimpots) after it has distrorted. Could be vertical size or vertical sync depending on the orientation of the screen.
If one section is darker - it will be the chassis. I had the same issue with one of my cabinets recently.
