I have no where near the expertise of many of you on this forum. I do have some practical experience with this issue. I own a 90s Bob Jackson that I love. It rides pretty nice and the colors are fabulous. I believe it is Reynolds 853. On serious downhills you are in for the ride of your life. The first time I rode it fast downhill I thought it was trying to throw me off. It shimmied something fierce. I ride with a more experienced group and I received the same advice, lean my leg agains the top tube. I was also advised that one most likely cause is the tendency towards building frames with a one meter wheel base. I am not sure about that. I saw some comments that a large frame bike with a high seat is one probable cause. I am not so sure about that either. I own a Rivendell Atlantis that is 64cm and the Bob Jackson is about a 58 - 60. My Riv will go down that same hill straight as an arrow and it is much bigger and higher. My guess is that there is most likely something going on like a resonant frequency of the frame being reached. A combination of components and the build of the frame and type of tubing combines to tune the frame to a particular frequency. Once that is reached you get oscillation. This is the same thing that allows sound to break glass. This might explain why one bike does it and another does not. Hey it is my guess but it makes sense to me. One thing for sure, I know it was not because anything is loose on the bike nor do I have the shakes. It is the bike and it only happens at certain speeds.
Ray Homiski
Elizabeth, NJ