AN MD80 CREW; UNABLE TO MAINTAIN FL350 DUE TO ACFT PERFORMANCE; DECLARED AN EMER IN ORDER TO RECEIVE IMMEDIATE DSCNT CLRNC.

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WE WERE FLT PLANNED AT FL310. DEPARTING PIT; WE WERE STEP-CLBED TO FL310. THEN ZDC CLRED US TO FL350 AND TO EXPEDITE ALL THE WAY UP. I SELECTED CLB PWR AND INITIATED A CLB AT THE RATE OF 1000-1200 FPM. AS EXPECTED; THE SPD 'BLED OFF' AS WE TRADED AIRSPD FOR ALT. APCHING FL350; WE ENTERED AN AREA OF LIGHT TO OCCASIONAL MODERATE TURB. WE LEVELED AT FL350 AT A SLOWER THAN NORMAL SPD; BUT STILL ABOVE CLEAN SPD FOR OUR WT. HOWEVER; WITH CLB PWR STILL SELECTED; THE ACFT WOULD NOT ACCELERATE AND THE AIRSPD ACTUALLY BEGAN TO DECAY TO BELOW CLEAN SPD. I DIRECTED THE FO TO GET CLRNC TO A LOWER ALT. BY NOW WE WERE WITH ZTL AND THEY COULD NOT IMMEDIATELY CLR US LOWER. I INITIATED A DSCNT SLOWLY IN VMC WHILE THE FO DECLARED AN EMER TO GET LOWER. SHORTLY AFTER DEPARTING FL350; THE ACFT ACCELERATED BACK TO NORMAL CRUISE MACH AND WE WERE CLRED TO FL310. THE REST OF THE FLT CONTINUED WITHOUT INCIDENT. WE WERE ADVISED TO CONTACT THE FAA'S QUALITY ASSURANCE DIVISION TO DISCUSS A POSSIBLE ALTDEV. I AM NOT AWARE AS TO WHETHER OR NOT A LOSS OF SEPARATION OCCURRED WITH ANY OTHER ACFT. HOWEVER; I AM CONVINCED THAT STAYING AT FL350 MAY VERY POSSIBLY HAVE RESULTED IN A MUCH MORE SERIOUS SIT.