Just today stripped a box with virtually no miles on it after it was rebuilt by someone else, the fault when it went to them was jumping out of fourth gear, the fault came back pretty much straight away - it's a syncro box and the cause is simply it has a four/five speed gear carrier (part of the case) in it instead of the syncro one so no way of preventing the main bearing turning in the case and wearing it so it then becomes loose and moves in the case

They had had a go at cinching it up by punching the case around the bearing but the alloy is too soft for that to work as the bearing race is so much harder and will out.
The pinion bearing was well worn and had not been replaced, the pinion lock nut wasn't properly tight and the diff bearings weren't set right allthough the backlash was correct so it was over loaded on one side of the diff and it had a mix of syncro rings fitted not one of which was the correct one on the correct gear.
It's probably a good job the fourth gear fault wasn't fixed cos if it had been driven much further it could have been an expensive bang type failure.
ps I know I'm not perfect but I'm trying to be and I'll honour the warranty
