308gt4 clutch or gearbox problem?

I have just taken the car for a quick spin tonight and I lost both 4th and 5th gear when I changed up from 3rd. I slowed down and got back into 3rd again and found both 4th and 5th gear. Not sure what is happening it happened twice I was not going hard on the gas but was not driving slow. Got back home and moved gearlever up and down the box 4th and 5th seemed very easy to select with no take up. Anyone got any ideas on where to start or have had similar problems. I do find reverse difficult to select sometimes this has only happened since engine came out for some work I am thinking it could be selection problems but not sure help needed. Thanks

 

I have a workshop manual but its not very clear on what to do in mine. I have tried searching on here but can't seem to find anything.