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I would check the gearbox shifter forks for wear, or even the drum shifter for wear plus the mechanism at the end of the gear shaft. Means you have to take it apart but on an old bike things do wear and if the box has been abused then thats your problem. Wheelies and some race bikes take a real tole on the gearbox. Just a suggestion but best I can come up with without seeing it. Try looking at this for an idea on the workings of the box.