I believe that the problem could be with the yoke on the steering column. There are 3 pinch bolts on the column. From what I understand, the method to correct an off center wheel (given that the wheels are already in alignment) would be to loosen the bottom pinch bolt (under the boot), straighten the wheel to the 12 o'clock position and then tighten the bolt back up. I know it can also be corrected by adjusting the tie rods or adjusting the toe, but many times when it is off center it's cause by the connection (pinch bolt) where the rack connects to the steering column slipping. Meaning the pinch bolt may loosen over time and the column slips out of the correct position with the rack.
This all from memory so hopefully memory serves me well.