As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ENGINE INTERMITTENT, ROUGHNESS, WAS BELIEVED CAUSED BY LEFT MAGNETO P-LEAD SHORTING OUT AGAINST OUTSIDE SHIELDING. BOTH LT AND RT P-LEADS INTERNAL INSULATION HAD DETERIORATED TO POWDER. THIS CONTRIBUTED TO ACCIDENT. P-LEADS APPEARED TO BE ORIGINAL EQUIPMENT ON AIRCRAFT.