I had it at the dealer for a different issue, and explained the shimmy symptoms. They checked over the front suspention and could not find anything to replace. The mechanic's best guess given the intermitten nature of the problem was that the caliper may have stuck enough those two two random times, and by stopping it unstuck. I've had sticking calipers years ago on an Acura, but they did not let go. I will check the wheel for excess heat if the shimmy happens again.
I replaced the front pad and rotors a year or two ago, but did not replace the pins the calipers slide on - I guess I could pull it apart and make sure it all looks okay.