Gearbox/clutch issue

The clutch stopped working earlier so I've ordered a complete set including master and slave, however sometimes when driving the transfer box jumps out of hi and won't select any of the hi/lo gears but with a play with the clutch and gears and transfer it goes back into hi, I'm after some advice on what could be causing it to jump out, it has done it on way home and with the clutch not disengaging I can't get it back in?
Any help would be gratefully appreciated
Dan