Sportsters have a "loop" in the spark plug wire!!
It collects moisture over time and corrodes..I had to change my Sportster Pug wires every other year!!
Just a thought..

 

I bought a 1981 sportster ten years ago and i had to ride it abou 400 miles home, halfway back it started spluttering and became unrideable by the time i got to the next motorway exit, luckily i had a friend abou 20 miles away and he rescued me with his van, it was late by then so we left the bike in the van and got some food and some sleep then in the morning the bike started and ran as it had before, on the second leg of the journey it did the same thing and over the course of time my investigations eventually (after changing coils, adding electronic ignition, etc, etc,) led me to realize that the wiring loom comes ver close to the rear cylinder as it goes to the trip switches (circuit breakers ) that some genius had decided to use instead of fuses, so i rerouted the wires to stop the copper wires transfering the heat and replaced the trip switches with a fuse box to stop the cutting in and out and she runs like a dream without all the erratic popping out of the exhaust and carb. I read a lot about sportsters giving trouble like this and have read and been told many imaginitive possible causes but never had anyone suggest that these troublesome breakers might be temperature sensitive and of course under test conditions they work perfectly.
I can understand how the manufacturers have difficulty trying to keep the wires away from the engine as there is no gap between the engine and frame at the back but needs must. I hope this helps to save someone the trouble that i had to go to, happy motoring.