[Ron Ramirez]

The old insulator (the thin, flat insulator, that is, placed between primary and secondary windings) was made of a nitrocellulose material (same as old stock film which self-destructs). As that stuff starts breaking down, it eats the insulation off the coil's primary winding, then starts working on the wire itself. If left alone long enough, it will also start negatively affecting the secondary underneath the insulator as well. This can cause the wacky resistance readings.