Problems

There are four likely causes for the gear shift problems:
1) the gear linkage is loose or incorrectly aligned - the "lollipop" linkage is normally the problem with this, just tightening it up will make it consistent but it may take some tweaking to get the alignment spot on.

2) the clutch is worn out - I won't say it is a simply fix but it isn't too difficult on an X1/9 and far better then most others

3) the lower engine/gearbox mount is worn out and (normally in conjunection with a bad gear linkage) is allowing the gearbox to change gear as the revs rise and fall and the engine twists in the engine bay

4) the 3 "detent" pins that lock the linkage in place have flattened out and are no longer capable of holding the selectors in position. This does require the transmission to be removed and opened up so not really something you want to do unless you have to and you are prepared to fix anything you find inside that needs replacing...

If you want to open the gearbox up I suggest you take a look at this series of posts on xweb:

http://www.network54.com/Forum/12159...+the+skinny---