I'd be inclined to point an accusing finger at the clutch first before blaming the gearbox. Is the clutch correctly adjusted? Too much free play? Where's the bite point? Is it a fair way through the lever's travel?

If the clutch is dragging even a little bit (not disengaging fully), gear changes at higher engine speeds will be clunky, particularly first to second.

Incorrect grade oil (too viscous) will cause similar clutch drag and subsequent gear selection problems.

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