Thanks for the correcting my misunderstandings. :king:I will add that when the shift stop lock nut comes loose, the shifter stop adjusting bolt will tighten down and second gear is generally where you will notice it most by popping out of gear. It pops out because the shifter stop prevented it from fully engaging the gear. Try backing off the shift stop for 2/4/R, or at least check that it has the proper gap.
More times than not, the problem is that the synchro teeth on the second gear have worn significantly and it can't lock in gear. For a T-5 the main thing that keeps a gear from popping out is the interlocking teeth on the ring of the gear and the matching teeth inside the slider. If you look closely, those two sets of teeth are "keystone" shaped and they interlock when torque is applied. That is what keeps them from popping out of gear. When these synchro teeth get worn and rounded off, they can no longer interlock and it will start popping out of gear when you apply torque.
There are three spring loaded "keys" or sometimes called "Struts" in the synchro hub. They help to hold the synchro slider in the neutral position. Once in a gear, they do nothing to continue to hold it in gear.
The spring loaded detent ball under the shift anvil (where the shifter finger inserts) helps a little to keep the shifter in place , but it can't do anything to prevent it from popping out.
So, a shift stop that is too tight, or bad wear on the second gear and matching synchro slider will do it.
And as said above, sometimes the fork pads give out or a fork can be damaged.