I have never been inside a coil to see what was shorting when they got hot. The old timers seemed to think it was shorting against the can. Maybe they short between the winds which actually sounds reasonable. But the bottom line is they throw a weaker spark when they get hot, and I have seen quite a few of them function quite normally when they cooled off. I have also seen them get so bad that the vehicle stopped. The vehicle was allowed to sit over night and the next morning it started as usual. By elimination, I still think the coil is the prime suspect.
