hi all, 1969 1600 GTL cortina type 3 gearbox that has a nasty habit of slipping out of 4th gear on over run only, not under acceleration. Is this hard to fix?

Easy Peassy!.............gearbox OUT......carry out Major Overhaul !

Bugger, i was hoping it was going to be a bit easier than that, thanks for the reply

Make sure your Selector Levers are all adjusted & tight if adjustable and you can get to them if you have side or bottom inspection plate. If top inspection; out she comes sorry.
 
There's one thing you can try if ur a gambler which is 99% sure of NOT working: drain the old oil out, esp if it's Synthetic or NOT standard, and refill with basic original oil...........I have seen improvement in at least one case

You could also check to see that nothing is pressing on the gear lever in top gear(carpet, rubber boot or body)
Next, check to see that engine and gear box rubber mounts are in excellent condition and holding everything tight.
Might get lucky and save some drama.

ok thanks, i will check the mounts, and start looking for another box