Percolation? Heat soak? Don't drive during summer??

Phreakish said
The previous owner who installed it probably didn't protect the wire along it's path, and now there's a big hole rubbed in the insulation causing the wire to partially ground. Partial or bad connection could cause repeated arcing. Kinda like a bad battery trying to crank which causes the relay to 'buzz' or repeatedly click...

That I understand. But the green wire doesn’t actually trigger the firing. The switch (points, light or magnet) does that.

EDIT: a quick thought just popped into my head. I suppose it is possible that a grounded wire can trigger firing. So DD may not be on the wrong track with that. My bad for not thinking that through.