What was found
AT FL340 DURING A CROSS FEED OPS TO CORRECT MINOR FUEL IMBALANCE THE LEFT ENG ROLLED BACK TO 52 PERCENT N1. LEFT MAIN BOOST PUMP WERE TURNED BACK ON AND X-FEED VLV CLOSED. ACFT RETRIMMED AND A/T ARM SW TURNED OFF. INITIALLY ENG WAS UNRESPONSIVE TO CHANGES IN THROTTLE POS. REFERRED TO QRH FOR ENG SURGE/STALL AS THROTTLE WAS RETARDED ENG INDICATIONS FOLLOWED NORMALLY. ALLOWED ENG TO IDLE FOR 2 MINS THEN SLOWLY ADVANCED PWR TO CRUISE THRUST. ALL INDICATIONS NORMAL. PERFORMED SUCTION TEST. ALL OK. REPLACING ENG FUEL PUMP AND PERFORMING HIGH POWER RUN. MAINTENANCE IN PROGRESS.