A B767 AT FL330 RETURNED TO THE DEP ARPT DUE TO CONSTANT 3%-5% FLUCTUATION OF R ENG N1. FELT IN SLIGHT YAW. AUTOTHROTTLE DISCONNECTED NO HELP.

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WE WERE SCHEDULED TO OPERATE ZZZ-ZZZZ ON JAN/FRI/06. APPROX 40 MINS INTO THE FLT; WE ELECTED TO RETURN TO ZZZ WITH AN ABNORMALLY OPERATING ENG. THE ENG CONTINUED TO OPERATE THROUGHOUT AND THE LNDG WAS UNEVENTFUL. AFTER REACHING CRUISE AT FL330; WE NOTICED THAT THE R ENG WAS NOT MAINTAINING A CONSTANT N1. THE AUTOTHROTTLES WOULD SET THE PROPER TARGET N1; AFTER A FEW SECONDS THE N1 WOULD FALL OFF BY ABOUT 3%-5%; THEN THE AUTOTHROTTLES WOULD RESET THE TARGET N1. THIS CYCLE CONTINUED TO REPEAT ITSELF. THE FREQ OF THE FLUCTUATIONS STAYED ABOUT THE SAME; BUT THE MAGNITUDE VARIED. THRUST LEVER STAGGER NEVER DEVELOPED AND THE TARGET N1 WAS CONSISTENT WITH THAT OF THE L ENG. YAW WAS SUBTLE BUT DETECTABLE; AND IN SYNC WITH THE ENG INDICATIONS. AT FIRST; I THOUGHT IT MIGHT BE AN AUTOTHROTTLE PROB; SO I DISCONNECTED IT; BUT WITH NO EFFECT. THERE WERE NO EICAS OR STATUS MESSAGES AND ALL CIRCUIT BREAKERS WERE IN. THE USE OF ENG ANTI-ICE HAD NO EFFECT EITHER. WE REVIEWED THE QRH TO SEE IF ANY PROCS APPLIED TO THIS SIT. I DID NOT FEEL THAT THE ENG LIMIT/SURGE/STALL CHKLIST WAS APPROPRIATE BECAUSE ENG LIMITS WERE NEVER APCHED; THERE WERE NO ABNORMAL NOISES; ALL INDICATIONS WERE CONSISTENT WITH THRUST LEVER POS; AND THE ENG WAS RESPONDING TO THRUST LEVER MOVEMENT. AFTER TALKING TO MAINT AND BEING UNABLE TO CLR THE PROB; WE ELECTED TO RETURN TO ZZZ. WE OBTAINED AN ATC CLRNC AND WERE CLRED TO ZZZ VIA THE ZZZZZ ARR WITH AN INITIAL DSCNT TO FL320. WE ADVISED THE FLT ATTENDANTS AND PAX OF OUR SIT. I FELT THAT THE SAFEST COURSE OF ACTION WAS TO OPERATE THE ENG NORMALLY. THE FLUCTUATIONS WERE NOT LARGE ENOUGH TO AFFECT ACFT CTLABILITY AND THEY CONTINUED REGARDLESS OF THRUST LEVER POS. WE HAD ALREADY BURNED ALL THE CTR TANK FUEL SO JETTISONING WAS NOT A FACTOR. WE WERE GOING TO BE OVERWT FOR LNDG; BUT I FELT THAT IT WOULD BE IMPRUDENT TO EXTEND THE FLT IN ORDER TO REDUCE OUR LNDG WT. THE DSCNT; APCH AND LNDG ON RWY 31L WERE UNEVENTFUL. DURING OUR TAXI TO THE GATE; THE FLUCTUATIONS PERSISTED; EVEN AT IDLE THRUST. ENG SHUTDOWN WAS NORMAL. WE DOCUMENTED THE ENG PROB AND THE OVERWT LNDG; AND BRIEFED THE MECH. I CHKED IN WITH OUR DUTY PLT IN ZZZ1. SINCE WE STILL HAD DUTY DAY AVAILABLE AND THE CREW WAS FIT TO SAFELY CONDUCT THE FLT; WE SWITCHED ACFT AND OPERATED THE FLT TO ZZZZ. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED A LCL MAINT TECHNICIAN SAID THE CAUSE OF THE N1 FLUCTUATION WAS CAUSED BY A LEAKING FUEL PRESSURE XMITTER. THE RPTR HAS NOT RECEIVED ANY RPT FROM MAINT OPS ON THE ENG OR ENG COMPONENT FAILURE.