If your gearbox jumps out of gear you need to change the balls and springs in the selector forks. Having removed the box from the car, it needs to be secured in a frame to hold it upright and accessible from both ends and top.

First step is to remove the cover and gear change gate assembly. With your fingers try moving each fork. If the spring is worn the ball will not lock the fork in the neutral or meshed position . The first step to removing the selector bars is to remove the nuts and locking plate at the back of the box. Next step is to drive out the selector bar with a drift. Care must taken not to lose the shims on the back of this shaft. As you remove the bar from the fork, catch the ball with a small magnet. This can be passed through from the back of the box as the bar is removed. Here you can see the hole in the fork for the ball and spring. Make certain you remove all of the spring if they have broken. See below - the picture shows old and new Springs and balls. Clean all parts prior to re-assembly with new balls and springs.