What was found
AT FL380 WHILE AT CRUISE POWER; APPROX ONE HOUR INTO FLIGHT; WE FELT A SHUDDER THROUGHOUT THE AIRFRAME; LOOKED AT THE #1 ENGINE INSTRUMENTS AND SAW EGT APPROACHING LIMIT; RPM ROLLING BACK. NO INDICATION OF FIRE OR LOW OIL PRESSURE. PULLED THRUST LEVER TO IDLE; INDICATION BECAME NORMAL. DESCENDED RAPIDLY INTO KGSP WHILE RUNNING THE ENGINE STALL CHECK LIST. WHEN THRUST LEVER INCREASED TO >50-55%; VIBRATION WOULD RETURN. LANDED WITH #1 AT IDLE AND NO REVERSE THRUST USED ON LANDING. / [01] #1 ENGINE UNSCHEDULED CHANGE REF; TT68Q005ZAZ8 FOR RIGHT. REF TTQ005ZBOM FOR REMOVAL AND INSTALLATION DOCUMENTATION S/N 891284 REMOVED MAN CFM56-7B24. S/N 891284 INSTALLED MAN CFM56-7B24. C/W ENGINE RUNS IAN MT 771-0001 NO DEFECTS NOTED.