Worn box?. can you get it in gear OK, and if you drop the clutch it just bangs out of gear? I purchased a C900 for parts that didn''t want to work in 1 or 2nd, so drove 60 mile in 3,4,5 but didn't have to stop, just slow at junctions etc, so OK ,Does it vary whether slow clutch or normal, or just made a bang as it jumped out. It may be the pinion gear that's worn3/4th, and so eventually that jumps out and locks the box. how long have you had it, was it like it when purchased, have you given it some right foot in first often?