I had the same problem with my bike. I had a steel cross fork that was flexing violently, so I replaced it with a Winwood carbon fork that seemed like it would be stiffer. That made a slight improvement, but the shaking was still bad. I messed with toe-in and tried different brake pads with minimal results. The thing that completely cured the problem was changing to a fork mounted brake cable hanger instead of the one mounted up by the stem. I'm not sure I understand why this works, but it really does. I'm not familiar with the fork on the Cannondale, but if it has a through-hole in the center of the crown just below the steer tube, you can mount a cable hanger there. The problem is finding one. I was not able to find one to buy, so I modified and old one I had from a mountain bike to fit the Winwood fork. It solved the problem completely. I believe that if I had done this in the first place, I wouldn't have had to replace the fork at all.