Two more items to add to my list of Internal stuff (just what you wanted to hear!)
Coming back to my comment on a worm Main Shaft Bearing and in line with that (Yes, I see you have a new bearing here), if when mounted and the retainer plate is tightened up the Bearing Floats (as it will do if over shimmed or the 'C' Clip is too narrow) it will have the same effect.
Going back a fair way now, I seem to have a recollection that you had burnt out the Baulk Ring on 4th Gear ?
If so, with these Gearbox Cases, this is usually from the Register in the Centre Web of the Gearbox Case for the Main Shaft Bearing being Machined too deep. From measurements I've made over loads and load of these cases, often they are in the region of 0.020" too deep. The pushes the Mainshaft too far forward, in to the 1st Motion Shaft and is what causes the Baulk Ring on 3rd or 4th gear to burn out. I'm not sure you had this issue (can't recall!) and if you did, what you did to correct it.
The other effect it will have is to remove all over-shoot from the Synchro Hub when selecting top gear and this will also be a cause for it to jump out.
One other cause is if the Baulk Ring on 3rd Gear is sitting too tall when fitted up. This will push the Inner Hub closer to the 1st Motion Shaft and not allow the Detent Balls to proper find their track, thus not full engaging and also leading to jumping out.
These Gearboxes have load and loads and LOADS of very small detailed items in them, all critical. Around 80% of the time, we can just replace the normal consumables within them - Bearings, Baulk Rings, Forks, Layshaft - without paying too much attention to them and happy days. But now more and more non-genuine parts are appearing on the market for them, we need to pay a little more attention to them - not to say Alex, that you've been at all careless, far from it.
A few years back, I did start writing a manual to cover the overhaul of these gearboxes, it is very much a work in progress, so far it's around 120 pages - and that only stripping and some inspection,,,,, I don't think I'll ever get there with it but I don't class myself as an expert with them, I'm only learning like the next guy and despite what I have seen and found over the years, there's still 'new' stuff that turns up,,,, oh the joy!