Sounds like an intermittent timing gear failure to me....

If you have the original fiber timing gear it is definitely on the way out.
I have had them slip on the hub, settle back into the right place, and repeat
the offense several more times, then die graveyard dead.

A new fiber timing gear is not too expensive, and if you do get a new fiber
gear I would recommend against changing the crank gear. You should replace the
brass spacer plate behind the timing gear, though.

If you opt for steel gears you will have to change both gears as they are
a different pitch.

Every B20 and B30 I have had has failed at least ONE fiber timing gear.
B16s, too.
--
George Downs, Bartlesville, Heart of the USA!