I had the same problem starting at 81 mph. I went to Butler Tire in Atlanta and they did a high speed alignment. Paid $130 and the problem is gone. I was told by less experienced shops that it is feathering of the tires (they must have seen a lot of the 350Zs).
You should look into getting an alignment done by a professional shop, not your run of the mill Firestone or local garage.
My 0.02.
Good luck on getting it resolved.