B777 FLT CREW HAS ALTDEV TO RECOVER AIRSPD DURING NORTH ATLANTIC TRACK ENTRY.

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PRIOR TO COAST OUT PORTION OF FLT; WE RECEIVED; CONFIRMED AND VERIFIED OCEANIC CLRNC TO CROSS THE NORTH ATLANTIC VIA ENTRY POINT SCROD FL390 MACH .84 ALL AS FILED. FMC INDICATED MAX ALT FL396 AT MACH .84 (ZERO FUEL WT UPDATE HAD BEEN CHKED AND VERIFIED DURING TAXI TO REFLECT CONCURRENCE WITH CLOSEOUT NUMBERS). CLRNC WAS ACCEPTED AND GANDER CTR CLRED US DIRECT SCROD CLB TO AND MAINTAIN FL390. AFTER AN UNEVENTFUL CLB AND LEVELOFF; RADAR SVC WAS TERMINATED AND WE WERE SWITCHED TO GANDER RADIO. AS WE WERE PREPARING OUR PAPERWORK FOR THE XING AND SWITCHING FREQ A QUICK INST SCAN REVEALED INDICATED AIRSPD DECAYING STEADILY TOWARDS MINIMUM CLEAN MANEUVER AIRSPD AND AUTOTHROTTLES COMMANDING MAX EPR. I ALERTED CAPT AND BEGAN A SLOW DSCNT TO TRADE ALT FOR AIRSPD. WITHIN 200 FT; AIRSPD HAD RECOVERED AND I LEVELED THE ACFT AT FL388. AUTOTHROTTLES ACTED ERRATICALLY AND WERE DISCONNECTED TO MAINTAIN AIRSPD; ADDITIONALLY ON THE FMC VNAV PAGE MAX ALT VARIED BTWN FL386 AND FL396. GANDER RADIO WAS INFORMED OF OUR INABILITY TO MAINTAIN FL390 AND RECLRED US TO MAINTAIN FL388 UNTIL ABLE TO CLB. AFTER APPROX 30 MINS; A SLOW CLB (100 FPM) WAS INITIATED TO MAINTAIN FL390. THROTTLES WERE OPERATED MANUALLY UNTIL THE FMC VNAV PAGE INDICATED A COMFORTABLE BUFFER ABOVE FL390 AT MAX AND THEN RE-ENGAGED. REMAINDER OF FLT WAS UNEVENTFUL; HOWEVER; AS A PRECAUTION (POSSIBLE INCORRECT ZERO FUEL WT AT CLOSEOUT) ALL MINIMUM MANEUVER SPDS WERE INCREASED BY 5 KTS.