- Try turning the brightness down on the deflection board. You may also need to adjust the contrast to compensate.
- If you still see the retrace lines even with the brightness turned down, it's a problem with the Z axis signal, which could be on the deflection board, or the game board.
If the monitor has never had a proper rebuild, it will need one. So that would be the next thing to do. (Cap kit, and reflow all header solder joints to start, then see where you're at.)