try using limits when you set walk designer.

 

for a slower walk try retiming the animation to a longer period. 

 

a neat trick to cure both is to just take every 4th frame for a cycle, save the individual frame poses.  then reapply the poses over the longer length.  will help to eliminate jitter and leave lots of room for improvement or to add to the walk.  (like reporter with mike...hand must remain in front ot the figure.  just key that into the single frame poses you save out.)

 

you might get some unusual effects from the empty frames with the curves in the graphs...if you are getting overshoot and something goes way out of line, go to the previous key and make the spline straight instead of a curve.  (or enter more keys inbetween, but that is the quick fix)

 

remember to leave inverse kinematics on for the feet or the feet will drift around on the floor or through it.  you do need to turn it off if you are going to use dynamic cloth...but in that case just fill out the frames for all the leg part, then the feet will remain firmly planted.