I've had that happen on three different bikes, one on which I attained 67 years ago mph going down a mountain pass. That one was a Trek 957. 20 years later, I experienced the wobble on the same bike while on an organized tour at about 40 mph. It was me, being older and getting a little more cautious. The other two times were on a brand new 7 and a Schwinn Letour. Same thing and each time it was a long straight. Have never experience it bombing down a canyon with lots of curves. I've since learned to relax and when I start getting a little tense about the speed, slow it down.