Have you tried hotter plugs? Once, I had an intermittent coil problem, when I was fouling plugs. I moved to a hotter plug and it eliminated the fouling issue and allowed me to continue diagnosing what ended up being a bad coil.
1999 California Motorcycle Company Softail Custom, S&S 88, RB Racing pro stock 2-1 exhaust. Super E with 32 and 74 jets. Daytona Twin Tec Ignition 1006. Wood W8 cam. New to me 2005 Electra Glide Classic, stock for now, but not for long...