Its likely the valve body on top of the ps box that is not centered properly... I've learned in the past... Lift the car (at least the front end), remove the wheels so there is as little weighted resistance in the steering as possible (or else, better yet, you could disconnect the pitman arm from the crosslink arm, thereby, truly eliminating resistance)... then, with the engine running, you adjust the valve body back/forth until it has become as near perfect as possible without its automated self turning. The key is to lift the car and remove all resistance, then adjust. You'll probably lose some fluid, so have a catch pan underneath.