1) Check the tires. What do you have on each bike? What PSI, and how old? Too soft of tires make the front end of a tandem feel insecure, and make it wallow and search. Too hard or just overinflated can make it feel twitchy by transmitting too many little bumps back. You need to adjust the PSI for how much you and your partner weigh. My favorites are Schwalbe Marathons.
2) Check your weight distribution. Try moving your weight forward or rearward on the bike (slide seats on their rails, adjust the handlebars). For me - bikes get twitchy when the weight is too far forward, and are searching and insecure when it is too far back.
Having a trailer make it worse makes it seem more like what I describe as wallowing - not twitchy. Normally trailers absorb little vibrations running through the frame - reducing twitch. At least with me they do - but everyone is different...
Can you swap the front wheels to see of the cannondale front wheel helps the Santana? I would suspect a loose steering stem or front axle problem before the forks...
Last edited by JustWant2Ride; 06-26-12 at 09:07 AM.