have you checked the gear linkage pivot that sits on top of the steering rack they are prone to seizing up and this causes it to jump out of gear(honest)if its not that 5th gear can nearly always,on most cars be taken out without even taking out the gearbox.it sits just behind the end cover on the g'box and in most cases is pretty easy to get to.!!
My quote does appear to be a bit on the high side. I'll have to find out from my mechanic quite why he's so expensive - normally he's quite reasonable.
He's found me a pukka fully-reconditioned 'box if I need one and that may be the expensive bit of the quote, and/or maybe he's just scared of the amount of time it may take to fit (he's not an Alfa specialist as such - he took one look at the V6 and went all white!).
I guess he'll check out the linkages and selector forks before changing the whole 'box.
Failing that I have a mechanical failure warranty when I got the car and even though there seems to be a million different ways to invalidate it ("the client must not tut loudly or look at the car funny if it fails") I MAY be able to touch them for some of the cost. eek!