don't turn the game on (meaning make sure the power's off), unplug the JAMMA harness. see the small 6 finger portion of the JAMMA edge after the key in the harness? if the fingers are covered in a lot of shit, that could create a bunch of resistance ensuring sufficient power doesn't make it to the components on the board. I've seen this happen a lot on the later model Midway boards for some reason..
the solution in many cases is to remove the board from the cabinet, and use a pencil eraser on the 6 fingers on both sides, ensuring you don't flex the board or damage any of the surface mounted components, and also that you keep the eraser shavings away from the board itself, you want to wipe them away with your fingers.
if you're handy with a meter, you can check power at one of the roms (while the game is powered on, and after you've plugged the JAMMA harness back in)... assuming the U-shaped notch on the rom is facing your left, put one probe on the lower right pin, and the other on the upper left (or flip this 180 degrees if the notch is on the right). you want to set the range to 20V DC. you're looking for about 5.10 volts at one of these roms, and no lower than 5.00.
if you have the power supply's +5 cranked and you're not getting sufficient power to the rom, then you're looking at the pencil eraser trick outlined above. I had a Midway Vegas board where the +5 JAMMA fingers burned off, I wound up having to replace that input board.
if you have questions PM me.