Sounds like a weak bendix (return) spring on the starter.... Lemme see if I can 'splain it in easy to understand language...When the starter first starts to spin, the gear moves outward (centrifugal force) to mesh with the flywheel, stretching the return or bendix spring. (Inside, or under the starter gear) When the engine starts and the speed of the flywheel is equal to that of the starter gear, the friction between the teeth on the two gears is reduced and that is when the bendix spring pulls the starter gear back away from the flywheel. (The friction between the teeth on the flywheel and starter gear is what basically holds them together until it starts) You should be able to get a replacement at (most) any parts store. Not that bad of a job, just make sure to disconnect the battery B4 removing the cables to the starter. Lots of stuff to ground out the energized cable on!!!

Not a text book description, but hope it helps.
MCTOY