I had this exact same problem with my C'dale touring bike, which came with an aluminum fork (instead of the steel fork the specs showed.) The bike was fine when carrying a load on the front lowriders, but shuddered like hell when unloaded. My LBS worked on this several times and never could get it fixed. They ruled out brake and headset issues. Cannondale was not receptive at all when I asked if whether this was because they used the ultra-stiff overbuilt aluminum fork instead of the fork that their specs showed for this bike. They also refused to believe that my LBS (the c'dale dealer where I had bought the bike) had ruled out the usual suspects. Frankly, they were rude and I won't by another Cannondale again because of this (i've had 4). The problem went away immediately when I swapped out for a Surly Long Haul Trucker fork.