I have pretty much the same thing in my house. i think the motors are
20 years ould now.

Very unlikely to have that many defective motors. I'd look to another
problem in mounting or gearing. My first guess would be the motor is
too small for the pulley gearing you have, check a new one with an amp
meter and see if its pulling more than name plate. If so, change
pulleys or go to larger motor.

Also possilbe that mounting is poor taking the bearings out.

I suppose you'de know it you have low voltage problems at your site.
Even occasionally kills motors.

Karl