You might have spark leakage under higher boost, from either the spark plugs, or the distributor cap.
Have you tried dielectric grease?
I bought some at autozone the other day, as I was getting leakage when I raised the pressure in my spark plug chamber.
Search: autozone AGS Dielectric Spark Plug Boot Protector 0.14oz
It's $1.69
https://www.youtube.com/watch?v=FBKAQjfEijs