What was found
(CAN) ON GROUND RUN, LT MAGNETO ON FRONT ENGINE HAD A 200 RPM DROP. DURING TROUBLESHOOTING FOUND NR 4 CYLINDER TOP LEAD SHORTING FROM INNER COIL THROUGH INSULATOR TO CAP. LEAD REPAIRED, AIRCRAFT RELEASED INTO SERVICE. AFTER A FEW HOURS OF FLIGHT, THE PROBLEM RE-OCCURRED ON RT MAGNETO FRONT ENGINE. REPLACED IGNITION HARNESSES WITH NEW. MAGNETO RT PN S6LSG-25T, SN D03GA258R, LT P/N S6LSG-25, SN D03GA179R.