I usually get around the bvh and the walk designer thing both in the same way: Start out by deleting big blocks of keys, or just save the 'important' poses they generate. Same result either way. bvh and walk designer both give us way too many keys which amount to nothing but clutter and cause problems. In fact, by creating keys at every frame they both completely eliminate the smooth tweening that your computer is capable of. Keep the important poses they generate and delete the unnecessary ones in between. For example, a walk cycle only needs two poses plus their 'mirrors' for a total of four. A lot of the time I just save two poses generated by walk designer to the pose dots, delete all frames and make the walk cycle from that. Only takes a few minutes. Usually very little clean up to do and no more jerkiness.