Allis had this problem on the C, CA, D17 to name some.
I can't be sure of why this problem occurs everytime.
I can relate what the deal was with my CA which had the 3rd gear jumping problem. The gears were as good as new.
The Gears are constant mesh so they should look good. The power is transmitted from the gear on the countershaft via
a sliding collar that looks like a syncro from a car transmission.
The knobs on the third geat showed some wear but not sufficiently
to cause a problem. The collar slid over these knobs cleanly and easily. I bought a used fork assembly and measured both of them before I installed the new used fork assy. The new one pushes
the collar toward the gear knobs 0.060 inches further than the original. Even so, I think that it is still 0.045 short of where it needs to be but it does work. You can't adjust the fork by bending because the fork also shifts another gear. Though I have not done this I believe the problem is in the shaft on the fork assembly. I believe the ball position is off and that a new
shaft exactly as original but with the appropriate detent pushed
over 0.060 to 0.080 will solve the probem. I intend to do this as soon as this project comes up on my list. Even though mine works,
I am not happy with the feel,