When the car is on the road it seems to be fine.

The power steering rack or the pump is going bad. If it only happens at the extremes of the turn at full lock, you will have more time to replace the part. Try checking the pump by grabbing the pulley and checking for play. If there is any replace it and seee if that takes care of the problem. Also replace as much of the fluid as possible to get contamination out of the system from the failed part.