Are you saying that the connection between the gantry and the leadscrews is loose? If so, see if you can tighten it, and use some Loctite to keep it from working loose again. If that doesn't help, try using different pieces of metal to connect the two parts more rigidly, and perhaps more screws. I think that would be better than replacing the whole mechanism with open belts, which aren't usually recommended for routers.