I like post #14 as well. Over time the rotor shaft end play may have increased due to wear. This causes it to occasionally drift away from the stator poles and skip over some from time to time. Try laying the clock face down so that gravity tends to pull the rotor in toward the poles. This may fix or improve the time gain. Laying the clock face up should have the opposite effect.
Rotor shaft bearing wear can cause the same effect by allowing more lateral movement away from a pole at just the right time to allow it to be skipped. You should be able to physically move the shaft to see end play and bearing play. Compare with a clock that still works.
Fixit
Rotor shaft bearing wear can cause the same effect by allowing more lateral movement away from a pole at just the right time to allow it to be skipped. You should be able to physically move the shaft to see end play and bearing play. Compare with a clock that still works.
Fixit