It's pretty well documented that the vast majority of gas engine glitching issues are blamed on the ignition. What seems to fail to get out after the fact is that the vast majority of what was called an "ignition induced" radio issue turned out to be another component that was either worn out, defective, or improperly installed. The RC Exl ignitions work very well when installed correctly, and it has already been noted that the plug cap was previously improperly installed. It also possible that the plug cap was damaged internally during an installation and removal process. If the internal cap spring has been stretched out it could be arcing.

Time to go through the rest of the flight system to verify that everything else was done correctly with quality components.