AN OCEANIC B757-200 FLYING AN INCORRECT MACH SPD WAS APCHED BY A RVSM ACFT 1000 FT ABOVE. DUE TO CONTINUED PROX ACFT OFFSET PARALLEL TRACK.

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ACR ACFT ON A REVENUE FLT. ACFT ASSIGNED AIRWAY ROUTING AT FL350. AFTER PASSING ETP ENRTE (1000 MI TO TRAVEL TO DEST) AS CAPT OF THIS FLT I NOTICED AN ACFT ON TCAS; APPROX 1000 FT ABOVE MY ALT APCHING FROM THE REAR SLOWLY OVERTAKING MY FLT. AS THE ACFT FLEW OVER MY ACFT I NOTICED THAT MY INDICATED MACH SPD WAS LOWER THAN MY ASSIGNED MACH NUMBER SO I RESET IN THE FMC MY PLANNED CRUISE MACH NUMBER. MY ACFT ACCELERATED AND WAS SLIGHTLY BEHIND AND L OF THE ACFT 1000 FT ABOVE MY ACFT. WHILE NOT UNUSUAL; IT SEEMED ODD TO ME THAT THE B767 ACFT 1000 FT ABOVE MY ALT WAS NOT PULLING AWAY FROM MY ACFT. I RECHKED MY INDICATED MACH NUMBER TO FIND THAT IT WAS NOW ABOVE THE TARGET SPD OF MACH .79. I RESET MY CRUISE MACH TO THE CORRECT SPD AND THE ACFT FINALLY STABILIZED AT THE ASSIGNED SPD. A CONTRIBUTING FACTOR IN THE SPD CTL DILEMMA WAS THAT THERE WAS LIGHT TURB IMMEDIATELY PRECEDING AND DURING THE TIME THAT THE ACFT PASSED ABOVE ME WHICH CAUSED THE INDICATED MACH SPD TO WANDER WITHOUT 'TIMELY' AUTOTHROTTLE INPUTS. I GOT A CALL FROM COMMERCIAL RADIO REQUESTING MY MACH NUMBER AND I RPTED MACH .79. THERE WERE NO FURTHER COMS REGARDING MY ETA OR SPD. THE ACFT CREW ABOVE ME WAS APPARENTLY UNCOMFORTABLE WITH MY CLOSE PROX TO THEIR ACFT AND TURNED L AGAIN OVER THE TOP OF MY FLT TO AN OFFSET COURSE TO WHAT LOOKED LIKE ON MY TCAS TO BE 1 MI. I LATER DETERMINED THAT THE ACFT'S DEST WAS THE SAME AS OURS. WHILE MONITORING THE HF RADIO AND WAITING TO PASS MY POS RPT; I HEARD THIS ACFT AHEAD OF ME PASS ITS POS RPT. COMPARING MY POS AND ESTIMATES MINE WERE WITHIN 2 MINS OF THE ACFT AHEAD FOR THE NEXT 2 POS AND MY PLANNED AND ACTUAL POS ESTIMATES REMAINED STEADY. THE ACFT ABOVE ME CHKED IN AT A COASTAL WAYPOINT WITH CTR ON VHF WHILE APPROX 18 MI AHEAD OF MY FLT. AN ISSUE IN THIS SPD DILEMMA WAS THE FACT THAT ON THE AIR TO AIR COM FREQ; THERE WAS AN EXCESSIVE AMOUNT OF CHATTER AS WELL AS INTERFERENCE ON THE FREQ AT THE TIME WHICH DISTR ME BEING ABLE TO RECEIVE ANY RADIO CALLS FROM THE ACFT ABOVE ME.