78 Sporto Trans - Jumping Out Of Third Gear

[AUS911SC]

Recently purchased a mint 78 SC with all the factory goodies you could think of, this unfortunatly includes the sporto box, a luxury option in its day but not the choice of enthusiast - beggers cant be choosers I guess. I will eventually upgrade to a 915 box but in the interim I'll make do with the sporto. So far so good however I am having a couple of small issues with the box these are 1) Falls out of third when driving below 60km/h, above 60km/h is fine or when accelerating in third is also fine 2) When engine / box is at normal operating temperature, reverse has a tendency of cruching when placed into gear. My question is has anybody else had this problem if so whats your diagnosis? Thanks in advance guys...