Try adjusting picture controls. If this does not work, it is possible that you are sending the monitor a signal at the wrong refresh rate. Read the manuals and see which rate it needs. If it needs 15Khz, you may need an ArcadeVGA card, or to play around with AdvanceMAME. If it needs 25Khz (EGA), you might be best with a Trident Blade card and advanceMAME, or possibly a scan converter.
Sending too high a refresh rate will DAMAGE the monitor if it is done too long.