I saw the clamp on thorusa.com, but it said it was for a double crankset so I didn't order it.

The first time I went down a mountain on this, which was last month on the Lake Tahoe side of Kingsbury Grade, the bike felt squirrelly at speed so I didn't go too fast. After that I lowered the bars as far as they would go. The next day's descent (Geiger Grade) was much smoother, so I was able to go as fast as gravity would take me.

I don't know if that would help your speed shimmy. You may want to give it a try.