67 johnny 20hp jumping gear?

[Steve Mahler]

i have a mint 67 johnny 20hp that runs great until I go to plane- at different speeds (rpms) each time, it feel like someone hits the clutch- slips and revs. The shear pin and prop are fine, and when this happens, I cannot see or feel any movement in the shift lever.

Can I assume 42 yrs of wear on the lower unit gears?