Mechanical linkage wear can result in the starter armature beginning to spin before the pinion gear is fully engaged into the flywheel. Check out the mechanical relationship with the battery cable disconnected first, to be sure the starter gear is fully meshed before the contact button is depressed. If that's OK, then investigate to be sure the flywheel and the starter are compatible. Also check to be sure there's not an area on the flywheel where the teeth are badly worn- - - -the engine tends to stop in one of three places due to compression, so there can be localized wear spots from repeatedly engaging the starter in the same places.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!