Be certain sickle is not binding in full length of travel even if all new parts.If you've got pitman drive instead of wobble box be sure bearing is tight and fits tight in bore on ecentric drive casting.The "yoke" must have NO play on ecentric casting taper pins.Be sure brgs on pitman pivot are tight.May also want to check brgs on shaft that holds ecentric drive for play.Some sort of stabilizer is holding cutter bar.No play in it either.Bottom line is you either have too much free play in system causing jerking or sickle is too hard to move.HTH