It keeps slipping out?

OK, I've pretty well got my 1975 88" 2.25 diesel Series 3 back together following a rebuild on new chassis and bulkhead, and now starting to address some of the niggles...

Some of these may appear pretty basic to the more experienced amongst you, but please bear in mind that this is the first time that I have ever done anything remotely mechanical!!!

First thing on my list (apart from the major lean to one side following fitting the parabolics) is that 4th gear now jumps out when not actually loading it with acceleration or driving. As soon as I throttle back and the engine is overrunning; 4th gear slips out?

This doesn't happen in any other gear, and never happened before I started the re-build. During the rebuild the engine and gearbox remained connected and were taken out and replaced in one lump...

Any ideas as to a quick fix, or is this likely something more major?

Thanks

I have the same problem time to trade her in fer a younger model

Apart fron Nathans suggestion could this be coincidence? All linkages adjusted up correct?

check the detents nuts on the selecters are tight, if they are you may be better off fitting some new detent springs in, the tension may have gone out off the old springs, thats all what holds the gears in.

when i rebuilt my gearbox i fitted the new springs in mine, i fitted top hat spring guides, made out of a round headed screws with the shanks cut down which put a bit more tension on the detents to help hold the selecter shafts in better, just made it slightly tighter to change gear.