Has anyone tried to find out if its the cylinders on the steering side (the ones on the gear itself) or the drive side (the ones at the servo) that are the problem?

If you purge all of the air out of the system and clamp the lines right where they come out of the steering cylinders, does the gear hold its position with sufficient force? Maybe try that with the nose gear straight and then off to one side or the other and see if you can taxi in a straight line or constant circle. If so, then the problem is on the drive side.

On the drive side, maybe you need larger cylinders to provide more force to the steering cylinders? Maybe the tube between the drive cylinders and the steering cylinders is expanding with the pressure too much and you are loosing steering effectiveness that way?

Bob