Had a similar problem myself. The walk designer seems to work best with figures that haven't been moved from the origin. In particular, I think that if you move the figure by moving its hip, rather than the body, the first step of the walk designer will re-centre the hip, and, as this is a spline keyframe, the first few frames will "rubberband" the figure about the scene. Also, make sure that the walk path starts at the right position and that IK is turned off. Best of lick.