Sorry can't help with the gearchange - maybe there's a vacuum leak somewhere? I can help with the spark plug issue though - You've got a serious problem there as that plug could blow out of it's hole and punch a dent in the bonnet, maybe causing a small fire. Is your engine misfiring? If you remove the HT lead from that plug and stick another plug in there and earth it on the rocker cover or exhaust manifold you should see the plug 'firing'. If there's no spark then check the HT lead & dizzy cap.
I had a similar thing happen on an old motorbike I had, with a cross-threaded plug. Tried to start the bike, engine fired up and POP - the plug shot across the garden at a million miles an hour!
Hope that helps