What was found
PILOT REPORTED LOSS OF ENGINE POWER AT 500 FEET AGL ON TAKEOFF AND AGAIN ON FINAL. PILOT MADE A SAFE LANDING. DURING TAXI BACK, MAINTENANCE PERSONNEL NOTICED ENGINE RUNNING EXTREMELY ROUGH. DURING TROUBLESHOOTING ON SUBSEQUENT MAINT RUNS, THE FUEL FLOW GAUGE SHOWED HIGHLY ERRATIC INDICATIONS. THE FUEL SERVO WAS REPLACED WITH ONE FROM ANOTHER ACFT OF THE SAME MAKE AND MODEL AND TEST FLOWN. ALL CHECKS WERE GOOD. AFTER UNIT WAS REPAIRED AND REINSTALLED ON THIS ACFT AND TEST FOWN, ALL CHECKS WERE GOOD.