My S2a gearbox (C suffix) was recently rebuilt and is now popping out of 3rd gear...

The gearbox suffered a broken mainshaft. I removed it (not a joyful task) and found a highly-recommended (though not Land-Rover specific since there's not one of those around here) manual gearbox shop. They tore it down and built it back up. I gave them the green bible and the Haynes manual for guidance and found them the parts they said it needed: the mainshaft and the needle bearing. I also replaced a missing synchro detent spring clip and the 3rd gear set since there was one very small chip in one tooth, but now was of course the time to do that and anything else that was amiss. They saw nothing else out of place.

I put it all back in (also not a joyful task) and drove for about 35 miles. All seemed fine. Then, for the first time ever and to my surprise, it popped out of 3rd gear when driving twice over the course of several miles. To remedy this I replaced the 3/4 shift fork detent ball spring with a new reverse detent spring (since they are thicker/stronger), which I shimmed a bit too since it's also shorter. This put more pressure on the 3/4 shift fork, which I thought would do the job. At the same time I removed the overdrive to look at the mainshaft nut, which seemed to me to be tight. I don't have special tool for this castleated nut so I did not loosen or tighten it but, again, it seemed good. The only thing I noticed there was that the bearing that sits within the forward side of the overdrive had some forward to back play, but this merely sits under the overdrive and would not seem to affect anything upstream...the overdrive is fine.

I drove it a short bit after this and it seemed to have solved the problem...yay...but apparently that's not the case as it's again popping out of 3rd gear when driving now...damn.

I've heard that popping out of 3rd might be due to the shift fork spring being too loose, a loose mainshaft nut, or worn 3/4 synchros but all of these seem to check out ok -- the spring is new (and thicker and tighter now), the nut seemed ok, and the shop mentioned nothing about the synchros being worn (nor did they look worn when I saw them through the inspection hole before the rebuild). Also, it never popped out of gear before the rebuild or even after many miles thereafter, as mentioned.

Ideas? Next steps? Thanks in advance to all of you who are gearbox-knowledgeable for any ideas...