Just got a car in to break, bloke had enuff of it, once the gearbox developed a fault, it jumps outof 2nd, 4th and reverse.

Took it down the road, get it into 2nd, and hold it there, its drives fine, no noise whatsoever

Now would this be the synchromesh, or just the linkage has become misaligned

 

as its 2nd, 4th and reverse-all gears on "bottom" of linkage i would say its the linkage mate, try another linkage up before stripping car.

hth

Tricky