You'll hear a lot of people complain that all cubs are squirrelly on the ground. It may not be a particular problem with your setup, but with the design of the cub itself. You'll typically need to be on the rudder as you add throttle slowly to keep the cub tracking straight during take-off. To minimize the problem, try adding a little toe-in on the main landing gear, ie the wheels point toward the centerline of the plane.