Sounds like a bent shift fork, which is damaging the C2-C5 gear slots and dogs. 
Do you think a bent fork is keeping the gears from fully engaging creating the bucking?
What happens is this: the one "good" dog that's usually left of the 3 (on C5) is having to take all the load. As the shaft rolls thru the gear teeth, it wobbles slightly from the sideways torque it now has, and when it reaches the side opposite the shift drum, the dog slips out, engaging into the next slot that comes around on C2. This cycle repeats until power is let up, or in real bad cases, remains until shifted to another gear.
On bikes with well-worn, but still operating well, gears, you can simulate this very thing by gently shifting partway into 2nd while turning the gearbox by hand (engine apart, no crank or primary chain, turning the mainshaft by hand). It's sort of my backhand way of checking the looseness inside the C5 gear to see if the dogs might need to be back-cut (we used to call this "pull cutting" because it pulls the gears together under load) needs to be done to make a good gearbox again.
