ENRTE FROM UVF TO ATL; FLT CREW OF A B737-800 CLB TO FL390 TO AVOID WX. DESPITE SATISFACTORY PERFORMANCE CRITERIA IN THE FMC; THE ACFT IS UNABLE TO SUSTAIN AIRSPD AND IS FORCED TO DSND TO MAINTAIN AIRSPD ABOVE A STALL.

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BACKGROUND: FLT FROM UVF TO ATL; ABOUT XA:20 ETE; WE'RE ABOUT 3 HRS INTO FLT APCHING FLORIDA FROM SE. TRW'S ALONG SE COAST OF FL AND COASTAL WATERS; CROSSING OUR RTE. WE USE PIREPS; ATC; WX RADAR; FMS; DECIDE TO TOP THE WX AT FL390; IN WEAK PART OF LINE. I SLOW TO TURBULENT AIR PENETRATION SPD (280 KIAS/.76 IMN). ACFT STABLE AT FL390/.76 M. SAT = 52 DEGS C; FMS MAX ALT = 39.9; FMS OPT ALT = 37.7; FUEL = 7.3/1.0/7.4; GW APPROX 136.3. WX AT FL390 IS VERY THIN CLOUD; GOOD RIDE (NOT MUCH BUMPING AT ALL). EVENT: WITHOUT WARNING (NO OBVIOUS CHANGES IN FLT CONDITIONS) AIRSPD STARTED DECAYING. AUTOTHROTTLE RESPONDED WITH MAX CURRENT CRUISE LIMIT. AIRSPEED DECAY CONTINUED FASTER; AIRSPEED APCHED MIN MANEUVERING SPD. I SELECTED N1 LIMIT OF MAX CONTINUOUS; AUTOTHROTTLE ADVANCED; BUT INSUFFICIENT THRUST. AIRSPEED DECAYED BELOW MIN MANEUVERING; AUTOPLT NOW UNABLE TO HOLD ALT. AUTOPLT DISCONTINUED; EFIS DISPLAYED 'BUFFET ALERT.' WE'RE WELL BELOW MIN MANEUVERING AIRSPEED; DSNDING; NO STICK SHAKER OR BUFFETING. ABSOLUTELY NO ICING ON WINDSCREEN; WIPERS; WINGS IS VISUALLY DETECTABLE. CAPT TURNS ON WING AND ENG ANTI-ICE -- WE'RE OUT OF IDEAS. HAND FLYING; UNABLE TO CONTACT ATC DUE TO RADIO TFC; ACFT IS IN SLOW DSCNT WITH ENGS AT N1 LIMIT; AIRSPEED DANGEROUSLY SLOW. AT 38450 FT; MIA ATC ACKNOWLEDGES OUR REQUEST FOR FL370 AND APPROVES IT. NO OTHER WORDS; NO TCAS; NO TFC; ACFT ACCELERATES AT FL370; FLT CONTINUES UNEVENTFULLY. WHY? 1) HIGH ALT WINDSHEAR???? 2) ATC FREQ CONGESTION IS NEAR DANGEROUS AT MOST TIMES; ADEQUATE UNLESS SOMETHING GOES WRONG. 3) FMS/EFIS PROVIDED NO WARNING OF SPD DECAY UNTIL AFTER IT HAPPENED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS AT A LOSS TO EXPLAIN THE LACK OF PERFORMANCE GIVEN THE WT AND BAL DATA PROVIDED FOR THE FLT. HE STATED THAT THEY CLBED TO FL390 AT THE RECOMMENDED CLB SPD BUT THEN REDUCED TO THE PUBLISHED TURB PENETRATION SPD OF MACH .76 IN ANTICIPATION OF TURB ASSOCIATED WITH THE AREA OF CONVECTIVE ACTIVITY. WHEN THIS REDUCED MACH WAS OBTAINED THE DRAG ASSOCIATED WITH THE HIGHER ANGLE OF ATTACK APPARENTLY EXCEEDED THE THRUST AVAILABLE FROM THE ENGS EVEN AT MAXIMUM CONTINUOUS SETTINGS. THE SUBSEQUENT DSCNT WAS THE ONLY AVENUE AVAILABLE TO MAINTAIN FLT ABOVE STALL BUFFET. HE STATED THAT THE LOWEST AIRSPD HE SAW WAS ALMOST THIRTY KTS BELOW MINIMUM MANEUVER SPD FOR THE ACFT'S CONFIGURATION AND WT. THIS IS; AGAIN; CLEAR EVIDENCE OF OPS ON THE BACK SIDE OF THE POWER CURVE FROM WHICH DSCNT IS THE ONLY MEANS OF RECOVERY. COUPLED WITH THE INABILITY TO COMMUNICATE WITH ATC IN A TIMELY FASHION; THE RPTR EXPRESSED CONCERN ABOUT THE CAPT'S RELUCTANCE TO SQUAWK THE EMER XPONDER CODE. THE CAPT'S DECISION TO INITIATE ANTI-ICE MEASURES WITHOUT CLEAR INDICATION OF ACFT ICING WAS ALSO OF CONCERN IN THAT DOING SO BLED AIR PRESSURE FROM THE ENGS AND LIKELY EXACERBATED THE THRUST DECREMENT.