What was found
ON RETURN NR4 ENG RPM DROOPED TO 13,600. WHILE REVIEWING CHECKLIST FOR POSSIBLE ENG SHUTDOWN, ENG DROOPED TO 13,450 AND BEGAN TO HAVE HORSEPOWER SURGES. ENG WAS SHUTDOWN AND SECURED. IAW SOP’S (PROP GOVERNOR WAS CHANGED AND ACFT GROUND RUN AND RETURNED TO SERVICE) ON NEXT FLIGHT SHORTLY AFTER TAKEOFF NR 4 RPM AGAIN BEGAN TO SERGE RAPIDLY WITH HORSEPOWER. ENG WAS SHUTDOWN AND SECURED. ONE ENG INOP LANDING PERFORMED. MX FOUND PROP CONTROL BRUSH BLOCK WORN TO LIMIT, BLOCK CHANGED AND ENGINE GROUND RUN SERVICABLE.