LT77 Working one minute, not the next!

Hi Guys,
I've been trailing round this site for a while and haven't found any that would explain my fault just yet!
My 1986 ex MOD LT77 keeps jumping out of gear. This can be in any gear, sometimes setting off from standing, sometimes slowing down. It jumps out of gear and wont go back in. You can move the gear stick around into all positions, high and low range and diff lock, and makes no difference.
By playing around however you can get it to select a gear again and your off like nothing ever happened. It seams to help throwing it between reverse and 1st and will go back in.
I've checked the grub screw in the selector and its tight as can be. I cant see anything else wrong at this stage.

Can anyone enlighten me with some more info before I drop the box out and delve inside. That seams to be my only option right now.

Thank you guys!

its why it jumps out thats most worrying, gear lever is probably sitting in a worn housing and bias springs way out

Remove and strip Ul be a month of Sunday’s working out what it could be

This is a common fault on classic VW’s, the only way to fix is to strip and rebuild. Worn hockey stick on those.