Maybe the spark finds it hard to get where it's supposed to go. I'd check out the cap and rotor, too, as the internal spark gaps (between the rotor and cap) erode and the center carbon button wears (and sometimes breaks). Also check for tracking (black burned streaks) and cracks inside the cap. I don't know what the stock coil throws, but I'd bet it's at least 35k-40k volts to jump those FAT spark gaps, and your standard type coil only gives about 15k or so.

Not responsible for advice not taken...