ACFT ENTERS MACH BUFFET AT ALT DUE TO PERFORMANCE LIMITATION.

Narrative

DURING OUR FLT WE WERE FILED TO AND RECEIVED CLRNC TO CLB TO FL430. AT CRUISE FL430 THE TEMP WAS FOUND TO BE WARMER THAN FORECASTED AND THE ACFT WOULD NOT GAIN SPD AS IT LEVELED OFF INFLT. AS THE SPD STARTED TO BLEED OFF; WE REQUESTED FL420 SEVERAL TIMES WITH SHANWICK OCEANIC CTA AND SEVERAL TIMES GOT A 'STAND BY' REPLY. AS THE FLT CONTINUED W FROM ERAKA THE TEMP STEADILY GOT WARMER TILL THE ACFT'S AIRSPD DECREASED TO JUST ABOVE STALL VERY QUICKLY. AT THE SAME TIME SHANWICK ATC CAME BACK WITH A RERTE CLRNC TO COPY; AT WHICH TIME THE MACH INDICATOR TAPE ON THE PFD UNIT FLIPPED FROM MACH SPD TO THE IAS TAPE AND A BUFFET WAS FELT IN THE FLT CTLS OF THE ACFT. AT THIS TIME; I NOSED OVER THE ACFT TO GAIN SPD BACK AND LEVELED OFF AT FL428 TO MAINTAIN CTL OF THE ACFT. AS PNF WAS STILL RECEIVING THE RERTE CLRNC HE DEMANDED A DSCNT TO FL420; AT WHICH TIME I AM NOT SURE WHAT WAS SAID BECAUSE AT THE SAME TIME A SECONDARY BUFFET WAS FELT ON THE FLT CTLS; WHICH I RESPONDED TO BY NOSING THE ACFT OVER IN A DSCNT TO FL420; AGAIN TO MAINTAIN CTL OF THE ACFT. SHANWICK CTL ASKED OUR ALT AND WE TOLD THEM WE WERE NOW AT FL420 AT WHICH TIME THEY HANDED US OVER TO ICELAND CTL; WHO INFORMED US WE WERE NOT TO DEVIATE FROM ALT WITHOUT AN ATC CLRNC. OUR FLT CONDITIONS AT THE TIME OF THIS INCIDENT WERE VMC WITH A TCASII TARGET AT OUR S POS 30 MI AT FL350. THE FORECAST TEMP AT FL430 WAS TO BE ABOUT STATIC AIR TEMP -51 DEGS TO -52 DEGS; BUT WAS FOUND TO BE ABOUT -47 DEGS (ISA +09) WHICH WAS TOO WARM FOR OUR ACFT TO SUSTAIN FLT AT FL430 AT OUR WT. I BELIEVE ATC WAS TOO SLOW IN ISSUING US A CLRNC TO A LOWER ALT AND AT THE SAME TIME WE DID NOT STRONGLY STATE OUR IMMEDIATE CONCERN TO MAINTAIN FLT AT FL430 DUE TO THE SIT HAPPENING SO QUICKLY. IN HINDSIGHT; AN EMER SHOULD HAVE BEEN DECLARED AND VACATING FROM OUR ASSIGNED FLT LEVEL SHOULD HAVE BEEN RPTED TO ATC ASA. THE SIT HAPPENED QUICKLY AND I WAS EXPECTING TO HAVE THE REQUESTED ALT. WE DID NOT GET TO RPT THE DEV AS SOON AS WE WOULD HAVE LIKED WHILE TRYING TO INSURE CTL OF THE ACFT.