the car drifted to the left and the steering wheel was aligned in this position?

1995 Mercury Grand Marquis mileage: 145,000. I had the Pittman Arm on my 1995 Mercury Grand marquis replaced. After it was replaced, the car drifted to the left and the steering wheel was aligned in this position. When I took it back to the dealership and told them that it was not like that before and they told me that changing the Pittman Arm would not create an alignment problem. Therefore, they aligned the car. Now it seems that I have a problem keeping the car straight. I am continuously adjusting the steering wheel to keep the car straight. As I drive down the interstate, all of a sudden, the car will go to the left or right which is somewhat scary. Could an improperly installed Pittman Arm create this situation? I have been told there is something called memory steer, could this also be the problem. Thank you for your assistance.