What causes my bike to jump in and out of gear while going down hill ?

 

If it is changing gear by it's self, worn dogs in the gearbox, or bent selector forks, or worn drum. If it just starts to freewheel at low revs downhill, the one way clutch, behind the centrifugal clutch, has gone. However because of the design of the kick start mechanism on the 300, the kick start will slip as well if that one way clutch has gone.