On March 11, 2022, Craig F in Conover, NC, USA wrote:
"The popping out of gear is getting worse. I’ve heard it said that 'I’ve done all of the quick fixes'. What, if any, are there? And if there aren’t any, how difficult are these gearboxes to rebuild? Any special tools required"?

No special tools required, but do look forward to some quality time with your gearbox.

BTDT. When mine was popping out of 2nd gear many years ago, it was the 3rd gear retaining ring (thrust washer) on the front end of the mainshaft that got loose.

Then on deceleration in 2nd gear, parts from 2nd gear forward on the mainshaft can move forward a bit, which will disengage 2nd gear from the sliding hub.

Solution requires pulling the gearbox out of the car for disassembly to put the mainshaft front locking ring back in place. You might expect to find the spring loaded locking pin and spring damaged or missing, so small cost for a few minor bits,and gasket set and maybe new seals and a few quarts of oil. Mostly sweat equity if you're willing. Otherwise rather costly to pay someone else to partially disassemble and reassemble the gearbox.