Road force balancing is the ideal way to eliminate the tires as a possibility. Out of balance brake rotors can be a cause, I've chased one on my own 380 SEC. Make sure that you have your steering box in the correct adjustment. This is not an easy thing to do on a 201 6cyl. but if its sloppy the spring setup thats used to compensate for wear sets up an ideal place for the steering to "bounce" left to right. Good Luck!
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