Again? The "dogs", the locking lugs on the sides of the third and fourth pinions (on the main shaft) are worn, and the forward shift fork is bent. The contact surfaces of the lugs are originally slightly undercut relative to each other so the force transmitted from the third pinion to the fourth holds the two gears together. Once they wear past the point where the undercut becomes reversed, they start forcing themselves apart under power. The pressure on the fork can't rotate the shift cam, but the fork will deflect far enough the lugs to run over the top of each other and then snap them back together when the lugs line up again, thus the jerking.
Has nothing to do with broken gear teeth, normally.