Check out and correct any loose joints on the linkage from box to valve, change the centering spring in the valve. Take the Follow Up linkage out, make sure the spring inside of it is free to move. The washers on the end of the spring can get rusted solid, especially on snow removal jobs. To adjust the linkage, moving the ball on the anchor away from the center pin reduces the arc of steering, moving it closed increases it. Adjusting the length of the follow up will center the arc to keep it from banging on the stops. Steering should stop about 1/2" off each stop. Adjusting the length of the box to valve link will Ballance the effort required to turn the steering wheel in each direction. Never Ever run the engine with the follow up or valve linkage disconnected. If the valve gets moved there is no follow up control and the box can get ruined. Now in addition to the steering cyl and center pins a loose deck can cause erratic steering.
Later Bob