[Walter_Middie]

Sounds like the disk inside the motor has a bad spot or it's dirty. If it was mine, I'd pull the motor and see what that disk looks like.

I always cringe at the thought of taking apart something that still works - or kind of still works. In the end, it's never as bad as it seemed to be. Bite the bullet, dig into it, and find out what the issue is. Once fixed, it will work correctly for another 40 years.