Two Times Can't be a Coincidence...Dart not Firing

There is no doubt that the mag pickup "could" cause such a situation

One guy I ran into believes that some of them the magnet deteriorates.

Moisture is CERTAINLY a factor. Gets into the windings.......dirt and chemicals deteriorate the winding insulation......cause intermittent internal turn-to-turn shorts and "just plain" leakage.

You can NOT necessarily show that with resistance checks. Only some sort of dynamic or output test would show that. Since there is no such data, a guy would have to develop a tester, and say, test "a bunch" (maybe 50 or 100?) "known good" pickups to get a baseline.