Sounds like a problem with the linkage. Its not uncommon for valvers to jump out of gear, and most often its actually the engine mounts that are causing it, when they are worn. Having said that the boxes do go occasionally but yours doesnt sound like it. Jack it up and get under it and look carefully at linkage to see if there is a problem. Pop off plastic cover from gearbox (it covers oil drain plug and where linkage goes into box) and have a shifty. However it could be your box, in which cases start scouring the scrappys.