I'd really bet there is some resonance occurring within the part itself around the specific RPM range that is being sustained with those very similar gearing and speed combinations. Could be induced by other engine vibrations or the just the magnet pass frequency at that specific RPM. Either the pulse generator is simply shaking itself to death, or it's shaking far enough to make contact with the flywheel. Either way, I would verify the cruising RPM that this is occurring at, and then regear or adjust sustained cruising speed higher or lower to avoid holding the engine at that RPM for extended timeframes. And I agree, carrying a spare on a long trip would be a good idea.