Just like in engine building, a gearbox needs careful and proper attention when building it up. It isn't a difficult job, and most competant DIY mechanics can make a decent job of it. But, just like engine building, it's the knowledge of what to look for prior to and during the build that sets the 'professional' builders apart. Hopefully resulting in a more reliable, perfectly functioning unit.
One generally over-looked, or completely missed, feature is where the layshaft is carried in the gearbox casing itself. The shaft merely runs in two holes bored in the casing - one at the drop-gears end and one in the centre web. Over a period of time, especially on the later A+ gearboxes that were more thrown together at the factory rather than carefully built, these holes can become enlarged through poor build up and incorrect thrust clearances and main bearing failure; the one in the centre web suffering the most damage. So before building your next box up with all those nice new straight-cut gears, selector forks, bearings, etc., first fit a layshaft - one with no wear on it - all on it's own. Grab the shaft between thumb and forefinger and pull up and down and side to side whilst observing the outer end of the shaft where it protrudes through that centre web. If there is more than the merest movement, find another casing.
In use, the main and layshaft gear clusters are continually pushing away from each other, so an elongated hole will allow the gears to push away further from each other at the first gear end - causing the shaft assemblies to run out of true. This will cause tooth damage, jumping out of gears, premature bearing failure, and anything else you can think of with straight cut gears not running perfectly true to each other.
The other jumping out of gear and premature component wear problem stems from the selector forks. The cost of new rod-change selector forks has increased dramatically over the past few years. So much so folk have taken to 'repairing' worn forks by squeezing the 'prongs' together largely because this was alarmingly graphically demonstrated in a certain specialist car magazine some time back as the way to sort the problem, but missed out on definitive guidelines for doing so. Fine, and it can work, but what MUST be understood is that every effort is needed to ensure the prongs are squeezed together evenly from both sides, otherwise the fork will not allow the selector hub to run true. It will be biased one way or the other, causing the balk ring to drag on one gear or the other. The balk ring wears out very quickly so engagement becomes a problem on that gear, and the dog teeth get a bashing on the other gear as the balk ring doesn't get to do it's job properly. If in doubt - it'd be far more cost effective to fit new selector forks!
Useful part numbers:
C-22A1738 Hi-grade layshaft pre A+ 4 syncro, single step
C-22A1739 Hi-grade layshaft A+ 4 syncro, double step
C-22A1731 Hi-grade layshaft 3 syncro
C-22A1741 Competition baulk rings
88G396 Layshaft bearing, 3 syncro (both ends) and 1st gear end 4 syncro
CHM141 Layshaft bearing, 4syncro 4th gear end (2 for Pre A+, 1 for A+)
13H9513 Layshaft bearing, 4 syncro 1st gear end A+
22G889MS Selector fork, 1st/2nd gear 4 syncro remote type
22A611ms Selector fork, 1st/2nd gear 3 syncro
22A610 Selector fork, 3rd/4th remote type 3 & 4 syncro
2A3492 Remote-type selector fork retaining bolt
DAM7425 Selector fork - genuine, 1st/2nd rod change
22G1855MS Selector fork - non genuine, 1st/2nd rod change
22G1856 Selector fork - genuine, 3rd/4th rod change
22G2595 Roll pin for rod change 3rd/4th selector fork to selector rod
22G2203 Genuine Rover standard baulk ring
22G2203MS Non-genuine standard baulk ring