gear selector cables do stretch and could require replacment (pain in the ass job) as a minimum adjust the gear selectors, Haynes explpains how.
Your gearbox mount might be shagged so its worth checking this and upgrading the bush for a poweflex one, either way you will get a really nice slick gear change after this.
The Getrag box is bombproof so i doubt the box will be at fault.
press the traction control button, if the light does not light up its a bulb problem or do a dial sweep.