hi ,just like to add if I may.try this =select first then remove the selector shaft from the selector arm ,you will have to tie the shaft up or simply remove,and test drive ,obviously you will only have first gear . now the selector arm cannot be acted upon by other forces ,if car jumps out of gear now ,then the fault must be associated with the gear box.if all ok then I would go for the gear stick ,most likely be non standard ,if its a short shift the design may not suit.
mech monster
October 2011