A friend of mine called me, asking me why his customer's pool pump capacitor keeps going bad every 3 to 6 months. I told him that it was possibly a surge or a loose return path. I haven't come across this myself.

Any ideas?

Check the centrifugal switch for binding or welded contacts.

If the cap doesn't get switched out of the circuit once the motor is up to speed, it will burn up quickly.

Also check the centrifugal actuator for the switch, If that binds up it won't open the switch.

And check the voltage rating of the capacitor too. To be on the safe side it should be about 1.5 times the nominal mains voltage.
For 230 Volts a 400 V cap.
For 120 Volts a 200 or 250 V cap.

Is this a PSC motor? The capacitor would remain in the circuit. Could low voltage cause excessive current draw and wear out the capacitor sooner?