What was found
(CAN) DURING CLIMB AT 37,000 FEET LT ENGINE N1 ROLLED BACK TO 80 PERCENT. IGNITION SELECTED ON&ENGINE N1 RECOVERED, CLIMB CONTINUED TO 39,000 WITH IGNITION ON & AT ALTITUDE SELECTED OFF - N1 ROLLED BACK TO 80 PERCENT. IGNITION SELECTED BACK ON & N1 AGAIN RECOVERED. A/C DESCENDED TO 35,000 WITH IGNITION ON & AT ALTITUDE IGNITION SELECTED OFF - N1 AGAIN ROLLED BACK, IGNITION SELECTED ON & N1 RECOVERED. A/C DIVERTED TO CYMM WITH IGNITION SELECTED ON. TROUBLSHOOTING CARRIED OUT, FAULT COULD NOT BE DUPLICATED ON GROUND. A/C RELEASED FOR TEST FLT&ON FLT SAME DEFECT RE-OCCURRED AT 40,700. FUEL CONTROL REPLACED