Gearbox jumps out of 5th gear

[Username]

Hi,
My 1.6td Type 25 has a gearbox problem, which if anyone could give me some advice would be much appreciated. The symptoms:

1. There's never any noise or other indication that anything is wrong
2. When cold the gearbox is fine, maybe for the first 1/2hr or so
3. After the first 1/2hr or so, and when in 5th gear continually for some time (say on a motorway) it keeps jumping out of gear
4. Other gears operate correctly at all times
5. By holding onto the gearstick it can be held in 5th gear, again with no noise or sign that anything else is wrong
6. The fault seems to be getting worse, it used to jump out every 5-10 minutes or so, but now almost immediately.

I'm planning some serious work over the winter, which will include a gearbox rebuild but would like to spend the summer using the camper and not pulling it apart.
Any thoughts?
Many thanks in advance

[Aidan]

worn mainshaft bearing retention in the gear carrier or wear in the bearing itself
if the bearing is very worn it generally grumbles, more under load than on the overun
heavy build up on the plug magnet says wear
holding it in gear wears the selector fork and the synchro slider and selector shaft

[Username]

Many thanks for your prompt and informative reply Aidan, looks like the rebuild might be sooner rather than later!