Forgot to say , also check that the gear change linkage where it bolts to output shaft is torqued up properly. If it's not, it can cause missed changes/what side MS mentioned to happen. Also will cause wear on the splines....which is not good.
So basically I'd be check chain free play, what I siad above and that the gear lever moves as it should. It could of course, be non of the above(but hopefully it is one of the above
). I just like to start off with the KISS principle
Ben