I also had a wheel weight that came off on my truck that caused a shimmy at a set speed(65-70) barely noticed it below that speed. Rebalanced and rotated tires was the cure.
Check your tie rods and ball joints. While I haven't had it happen on a fox or sn car, I've had others that shimmied with one or all of the above in bad shape.
I have a bump steer kit that's been on there about five years or so. Other than that, I replaced both ball joints a couple of months ago when one of them started making noise.
Maybe now would be the time to upgrade to a bolt through kit.