I've had this same problem for a while. things i swapped to find problem, struts, center link and tie rods, strut mounts, control arm bushings, sway bar bushings,steering box and idler arm, just about the whole front end. I know my steering coupler is bad, but cant fix it till i fix the power steering. the only thing that gives a temporary fix is new tires, but they dont last long. I always thought the car had serious camber issues so i replaced everything and still the same. Maybe my frame is tweaked. Could the steering coupler cause this. Im running out of parts to change, maybe wheel bearings are the culprit?????