Valves sticking in the steering gear. You might try draining the fluid out and install some conditioner and new fluid. Just remove the lines at the steering gear ( not the pump ) and drain with the cap off the fluid resevoir. You will have to bleed the system by turning the wheel left and right ( but not all the way in either direction ) and keeping the fluid full all the time. After bleeding, center the wheels and let the engine run for 5 - 10 minutes. Recheck and top off fluid as required.
If this fails to free the valves, then replacement is necessary.