If the rims and brake shoes are new they just need a chance to break in a bit. I put together a Crosscheck with a Winwood carbon cyclocross fork and Avid mini canit's. When I took my first test ride the thing shuddered so severely that it was unrideable. But everything worked out in the end and it's an excellent set-up. Well, I did switch over to V-brakes. But that's another thread, eh.

Take some medium sand paper to the pads and toe them in then just take it easy for a few miles.

DanO