if it's the original monitor with the cabinet it's probably a Wells-Gardner K7500. I've seen this particular symptom on several U5000s and K7500s, it's just a bad design on the neckboard where the color drive transistors get very hot and it leads to cold solder or even worse yet, lifting traces.
so that will cause your smacking the cabinet to bring colors back.
ordinarily what I would do is DESOLDER the transistors completely out, clean all the solder pads up, then reinstall them. if you have lifted traces, then that's where it gets complicated. I've taken brand new transistors and without cutting the legs off them, folded them onto scraped traces (so you can apply solder to them) and make it work that way. the folding onto the trace will additionally hold it down so it won't move.
you might have to send it out for repair or find someone local to you that can do such a thing.
even if it's not a K7500 and it's the Neotec uh, maybe 25E or 2501, the same fix applies. color drives just get hot and the solder joints break down. just hitting them with fresh re-melted solder can usually take care of it.