Pull the starter off, look through the mounting hole, and turn the engine to inspect the flywheel ring gear. it sounds like you've got a damaged spot on the ring gear that's stopping at the starter location occasionally. Either replace just the ring gear on a standard shift flywheel, or change the whole flex plate if you're running an automatic trans in that case. It's also possible the solenoid linkage could be damaged or out of adjustment, allowing the starter to begin turning before the pinion gear is fully engaged into the flywheel on occasion.
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!