Did they replace the large silver can capacitor on the monitor? That's the main filter, it's actually a multi-section cap, several capacitors in one. When they go bad, the picture gets this wave through it.
You can isolate the problem to the monitor by feeding in some other video signal, or feeding the game's video elsewhere. Black and white arcade monitors actually just take regular composite video. The molex plug carries AC power to the monitor, as well as the video signal. Check the wiring diagram in the manual to see which pin is which. There will be a video line, and a video ground. You can make up a lead to go from that plug to an RCA connector. Center pin is video, outer ring is video ground. Plug that into a the composite video input on a television set, and see the game's video on that screen. Or, patch the signal from something else (like a Nintendo) into the arcade machine's monitor. This will help rule out if it's a board problem or a monitor problem.
For a test lead, a length of two conductor wire with some bits of a paper clip soldered to the end work. You can jam the paper clip pins into the back of the molex connector that goes to the monitor. To test the game's video on something else, unhook the molex from the monitor. To test some other signal on the monitor, plug the molex into the monitor, and unplug the edge connector from the game board.
-Ian