Reluctance to go from 4th to 3rd, 4 speed diesel box

[discipleofsketch]

As the subject sais, my diesel 4 speed is occasionally reluctant to go from 4th to 3rd. The workaround is to pause in the neutral position, push over to to the left as if i'm going to change into 1st or second, let it come back, then it will continue into 3rd no problems.

I can live with this niggle, but i'd be interested to know if this sounds like something that could maybe be sorted out with some adjustment in the linkage? Or does this sound like it's something internal to the gearbox (in which case i'm not going into that mystery box of black magic )

[Aidan]

If the gear linkage is all in good condition, adjustment not usually an issue with 3rd 4th as that's the gearbox's default position, then it could be a box problem, but a really sloppy linkage bush could be the fault -
otherwise
possibly a broken selector fork bearing - if you have changed the oil recently was there a piece of spring steel or any ball bearings on the drain plug magnet ?
possibly the 3rd4th synchro hub breaking up - worse when hot ?

[discipleofsketch]

I think it might be worse when hot - gearbox fluid hasn't been changed under my ownership (1year), but it is on the list!