Unlikely a gear could slip just barely like that. Generally if they shear a key the spin and do nothing.Have you thought about the chain of gears that drives your lead screw -- You have the headstock end gears feeding into the input of the gearbox. Then gears and shafts in the gearbox. If any gear in the drive chain is somehow slipping a bit on its shaft -- that will give you a finer pitch while it's slipping. Is that possible? Could a gear be missing a key, and it's just tight on the shaft but slips a bit here and there? Or something similar?