Try different things, but my guess is that you will not be able to eliminate it. And I don't see how one can enjoy mountain riding knowing, that they don't know, what will happen on the next descent. There will always be that grain of fear at hand - that the bike might start the death wobble. Staying so attentive to it will likely have the rider slowing down or not putting full attention to the descent and enjoying it. While I respect Cannondale and wish them the best, I would not keep a bike that I could not be comfortable with. The comment made by Ramon to just clamp the top tube is totally unacceptable.

The System Six that you appears to be a relatively new aero bike. The newness concerns me, as I wonder just what testing they have done. Wait till more feedback makes it way across the net, and you might find that you are not the only one experiencing this. Cannondale should take this feedback and immediately get it resolved (much like Calfee did a number of years ago), whether the culprit is their frameset of some third party component issue. Stepping to the sidelines, and saying to clamp the top tube is enough for me to eliminate consideration of a System Six in the future, maybe other models too. However, if the bike was going to only see flatter terrain, it probably wouldn't bother me....probably.