4th gear problems

[67-2door]

The gearbox on my 1098 minor has occasionally jumped out of 4th gear on the over-run since I've had it (only a month).
I changed the gearbox oil at the weekend (20/50 engine oil) and there was quite a lot of metallic swarf & grot in the hollow drain plug which obviously I cleaned out.
Since changing the oil the jumping out of 4th gear seems worse if anything, not every time but quite regularly it pops out of 4th into neutral when you shut off for a corner, also it sometimes trys to jump back out of gear as you change up from 3rd to top.
All other gears are fine, is there anything I can do or adjust to sort this or do I just live with it and save up for a replacement gearbox?
Thanks.
Neil.

[Kevin]

do I just live with it and save up for a replacement gearbox?
Thanks.
Neil.

I hate to say it but start looking, but there are good used gearboxes out there for reasonable prices.

[bmcecosse]

Probably caused by worn bearings in the box - start looking for another. For that car - it needs to be the 'ribbed case' box.