[capri 72 gt]

hi guys , My 72 capri gt v6 is fitted with a non original gearbox replaced with a type E 3 rail box from the later 3ltr capri \granada ,better shift and slightly better ratios. The problem I have is that the gearbox pops out of 2nd gear when changing back from 3rd to 2nd gear whilst driving spiritedly. only on downshifts not upshifts.when driving normally no problem only when pushing on. Before installing , the gearbox was checked by mechanic , as ok ,appears new syncros had been fitted and gears look ok ..Anyone have any idea's.Is it possible that the problem could be the gearstick connection to gearbox ?? Or is there anyone out there with experience with these g\boxes {40 years old**.cheers