B757-200 SUFFERS ERRATIC OPERATION AND EXCESSIVE VIBRATION FROM LEFT ENGINE DURING TRANSOCEANIC FLT. DESCEND BELOW RVSM AIRSPACE AND DIVERT TO NEAREST ARPT.

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OVER 3 HRS INTO OUR FLT AT FL360; ENRTE TO ZZZ; WE SUDDENLY EXPERIENCED A SIGNIFICANT LOSS OF THRUST AND YAW ASSOCIATED WITH A SIMULTANEOUS CLUNK SOUND COMING FROM THE L-HAND SIDE OF THE ACFT. LOOKING AT THE ENG GAUGES; WE OBSERVED A HIGH ENG VIBRATION IN THE AMBER CAUTION ZONE FROM THE L ENG. I IMMEDIATELY INSTRUCTED THE FO TO DECLARE AN EMER WITH ATC AND INFORM THEM OF OUR POS AND INTENTIONS; AS I SIMULTANEOUSLY DID THE SAME ON PRIMARY EMER FREQ 121.5 AND EXECUTED APPROPRIATE PROCS DEPARTING THE TRACK AIRWAY. I ALSO RETARDED THE L ENG THRUST LEVER AND REQUESTED THE ENG VIBRATION QRH FROM THE FO. IN THE PROCESS OF EXECUTING THE QRH ITEMS; THE ENG VIBRATION MONITOR INDICATED IT WAS NO LONGER IN THE CAUTION ZONE AND HAD STABILIZED AT ABOUT 1.7 WHICH LED ME TO CONCLUDE THAT EVENTUAL SHUTDOWN OF THE ENG MIGHT NOT BE NECESSARY AS ALL OTHER ENG PARAMETERS WERE INDICATING THEY WERE NOMINAL. AS WE DSNDED BELOW RVSM; ATC ISSUED US AN AMENDED CLRNC DIRECT ZZZ; WHICH WE WERE ALREADY AT A CRUISE ALT OF FL200. I HAD ALREADY BRIEFED THE FLT ATTENDANT AND PAX AND WE CONTINUED TO KEEP THEM INFORMED AS I CONSULTED WITH MAINT AND DISPATCH. WE CONTINUED WITH NORMAL PROGRESS; PLOTTING AND POS RPTS KEEPING DISPATCH INFORMED OF OUR FUEL AND FLT PROGRESS. WE LANDED IN ZZZ WITHOUT FURTHER INCIDENT OR NEED FOR INSTRUCTION TO BRACE BUT HAD THE GND EQUIP ON STANDBY JUST AS A PRECAUTION.