DC9 CREW UNABLE TO CLB TO PLANNED CRUISE ALT; DUE TO DECREASED ACFT PERFORMANCE.

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DEPARTED JAX ENRTE TO DTW. CLBING TO FL330. WE WERE HVY AND THE OUTSIDE AIR TEMP WAS WARMER THAN NORMAL (RAM AIR TEMP AT FL310 WAS -18 DEGS C). I WAS THE PF AND TRIED TO MAINTAIN 1000 FPM CLB. WHEN THE INDICATED AIRSPD DECREASED BELOW 250 KIAS; I ADJUSTED THE AUTOPLT TO COMMAND SLIGHTLY LESS THAN 1000 FPM CLB. PASSING APPROX FL310; THE AIRSPD SLOWED BELOW 240 KTS; AND I COMMANDED THE AUTOPLT TO CLB AT 500 FPM. PASSING APPROX FL320; AIRSPD SLOWED TO 230 KTS; AND I COMMANDED AS SLOW OF A RATE OF CLB AS I COULD ON THE AUTOPLT. SOON AFTER; I HEARD A VIBRATION (NOTE: I DID NOT 'FEEL' IT AT THIS INITIAL ONSET). I THOUGHT TO MYSELF 'COULD THIS BE A BUFFET; OR IS IT THE OVEN IN THE GALLEY BEHIND ME MAKING THE NOISE?' IT SOON BECAME APPARENT IT WAS A BUFFET. BY THIS TIME WE WERE AT FL328. I COMMANDED LEVEL FLT ON THE AUTOPLT AND TOLD THE CAPT 'I'LL TWEAK THE ALT WHEN WE PICK UP SPD.' I NOTICED THE CAPT HAD HIS HAND ON THE CTL WHEEL AND I ASKED HIM 'DO YOU WANT THE ACFT?' HE REPLIED 'NO YOU HAVE THE ACFT.' I REALIZED THEN THAT HE HAD HIS HAND THERE FOR THE PUSH-TO-TALK MIKE BUTTON (NOT TO TAKE OVER CTL) AND WAS TRYING TO GET A WORD IN TO ATC TO REQUEST LOWER. SOON AFTER; THE CAPT COMMANDED 'TAKE IT DOWN TO FL310 NOW.' I DISCONNECTED THE AUTOPLT AND GLADLY COMPLIED WITH HIS REQUEST. CAPT GOT THE WORD IN TO ATC REQUESTING FL310 AND WAS GRANTED FL310. ACFT INCREASED SPD; BUFFET STOPPED; WE LEVELED AT FL310. LESSONS LEARNED: NEXT TIME I DON'T GET THE CLB PERFORMANCE NEEDED TO MAINTAIN 1000 FPM CLB AT TARGET/EFFICIENT CLB SPD; I'LL BE QUICK TO ASK THAT WE GET ATC'S PERMISSION TO CLB AT A LESSER RATE. EVEN THOUGH OUR AIRSPD WAS ABOVE THE 1.3 BUFFET CHART SPD FOR OUR WT; THE PERFORMANCE WAS LESS THAN NORMAL. HINDSIGHT TELLS ME THAT THIS WAS DUE TO THE WARMER THAN NORMAL TEMP FOR THE FLT LEVEL. AGAIN I SAY; WHEN I'M IN AN AIRPLANE THAT IS NOT PERFORMING AS PLANNED; I WILL BE MORE AGGRESSIVE IN SLOWING THE RATE OF CLB (LEVELOFF IF NECESSARY).