Gearbox jumps out of third at 50mph

[RedWhiteFB]

Howdy all,
While I was driving down the South Gippsland Freeway today at about 50mph, the gearbox jumped out of third gear - back into neutral - without any intervention from me. Scared the beejeezus out of me. I put it back into gear and all was fine... until it happened again, and again... Eventually, I had to hold the gear stick down (column shift) and if I felt it start to slide out of gear, I could take my foot off the accelator and it would drop back into third.

Can anyone shed any light as to what would be causing this; and what I need to do to fix it?

Thanks,
Aaron.

[ash]

Mine did the same thing so I stripped it down.
I found that the mainshaft was worn out (chrome worn away down to base metal) and the bearings had a ton of play.
I rebuilt it with the kit from Rare Spares and it has been perfect ever since.

[RedWhiteFB]

Thanks for that Tim and Ash. So basically I need a gearbox reco. I'll check the bolts - but I don't think the gearbox has had any work done on it for at least 15 years - so I guess a bit of wear and tear is allowable! I'll just drive at under 50mph for a while; until the boss gives me permission to rebuild the gearbox.

Cheers,
Aaron.