Gear trouble with 3511

Hi,
I have a 3511 and the gears get stuck in reverse gear when working it a bit hard. So far I fixed it by taking the top of the gearbox, jiggling it around until the selectors are realigned.
Very time consuming.
Is there an obvious reason for it to reoccur?
Is there an easier way to get it back to align, preferably without having to take the lid of the gear box.
Thanks for your help.
Cheers Ziggy

In front of the hi/lo range lever and the PTO lever is a rowof 5 bolts. they have a spring loaded ball inside, that make the clicking sound when shifting. When you undo one, you can put the corresponding shifting lever back in place and get going again.
You can also undo the 4 M8 bolts of the shift lever column and put it back in neutral from that end.

What i did is weld the wear grooves in the gearbox housing... Not entirely because any top would cause the fork sliders to jam, but just enough to prevent this problem from occurring.

Hi Renze,
that worked a dream.
I managed to fix it in no time.
Thanks for that.

Cheers Ziggy