it could be the gears in your rack binding up, but its likely the pump. They wear out over time, just like any other part on the car. I would start with the pump since its the cheaper and easier thing to do. you can lift up your car and shake down the front end to see if anything has any play. if the gears in the rack are worn out, then you will have lateral movement on the tires when you shake them.
does the pump make any noise when it gets "really hard" lmao