why would magnetos go bad?

I have a sears garden tractor mod#917.273641. Last fall the magneto went bad and i replaced. It has been running good untill last month and the mag was bad again. I replaced and it was running fine untill today. It lost power and quit again. The mag is open from the small black wire terminal and the plug wire. The black wire terminal is shorted to ground. The mags came with a paper card that said it was 20 thousants thick and I place it between the armeture and the fly wheel. The mag part number is 492341.