Have someone hold the stomp pedal down while you tap the back end of the starter with a hammer. Don't overdo it- - - -just a moderate thump back near the brush holders. It's possible that the brushes are worn enough they're losing contact with the armature. I once suggested that to an assistant principal at a school where I was teaching auto mechanics. When he finally gave up and had the vehicle towed to my shop, there were a bunch of big dents in the windshield wiper motor!
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!