Is the center hub the main shaft with the drive gear on it? Not sure which part the center hub you mention is (just trying to clarify terminology so we are all talking about the same things). Assuming the main shaft and hub have been changed I would next look at the feathering shaft (goes through the rotor hub for the blade grips to ride on) and the blade grips themselves. The shaft needs to be straight and the blade grips need to rotate freely on their bearings with little free play (pulling the blade grips away from each other and then pushing them back towards each other should not reveal a large amount of play). Also have you checked the o rings in the center hub?