[Jac Mac]

Usually assuming shifter is correctly indexed it will be worn input or mainshaft bearings allowing too much fore/aft movement ( this is induced by the gear teeth helix). Once they have jumped out of gear a number of times the flanks of the engagement dogs/teeth start to form a 'track' on the 'back taper' and they need refaced or replaced to prevent it. One possible 'fix' before you remove the trans is to remove the 9/16" bolt between the 3/4 & Rev shifter boss's on the main trans housing and pull the detent spring out, stretch that approx. 1/8" and refit, You should note a slight increase in the effort to engage 3/4 gears and this 'MIGHT' be enough to deter the problem, but only if the engagement dogs/bearings are in good shape.