Hi Sean,
Have you had a look at your magneto?
I assume its new but I"d be checking the points for burning and arcing. The internal capacitor is serviceable and connected. Making sure nothing is loose inside.
Magnetos produce a lot of energy, if there is something not quite right, it could be producing extra electrical "noise" or arcing over inside the cap.
Edit:You don't mention P leads are shielded. P lead grounding... from Aeroelectric
http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdfmagnetos and the magneto switches. Spark plug wires and magneto p-lead wires are the ONLY wires in the airplane which are ALWAYS shielded. Further, p-lead shields should be grounded to the airframe/engine only at the engine end. The shield is used to provide a ground for the "GRD" terminals at the switch but no other connection to the shields should be made at the instrument panel end (I assume Bob is referring to a standard aircraft key switch)
Cheers
Tony