74 duster pulls to the left, then centers, HELP!

My 1974 duster with a 318 and power steering will pull to the left after making a left turn and continue to pull til you hit some sort of bump, then it straightens out, meanwhile you are correcting so the car swerves to the right, very dangerous cuz it can cause the car to erratically change lanes. I have changed my power steering box, and it didn't help. I really want to drive my duster! Anyone have any ideas? Thanks