POTENTIAL CONFLICT ENSUES WHEN A B757 IS UNABLE TO MAINTAIN ITS ASSIGNED ALT OF FL390. ALTDEV ALT EXCURSION. NON ADHERENCE TO ATC CLRNC. OPPOSITE DIRECTION TFC.

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CLBED FROM FILED CRUISE ALT OF FL370 TO FL390 PER ZDV REQUEST. TFC WAS GIVEN AS REASON FOR CLB; CHKED CURRENT WT OF ACFT AND MANEUVER CAPABILITY OF ACFT BEFORE ACCEPTING CLB. UPON REACHING FL390 ACFT DID NOT ACCELERATE PROPERLY; APPEARED TO BE HEAVIER THAN COMPUTER SHOWED. CONFLICTING TFC WAS AT FL370; 1 O'CLOCK POS; AND VISUAL. AFTER LESS THAN 2 MINS AT FL390 THE ACFT EXPERIENCED A VERY SLIGHT LOW SPD BUFFET; A REQUEST WAS MADE FOR A LOWER ALT. WHILE THE DIALOGUE BTWN MY FO AND ZDV WAS TAKING PLACE; THE SECOND LIGHT BUFFET WAS EXPERIENCED. I TOLD MY FO TO NOTIFY CTR THAT I WAS LEAVING ALT AND COMMENCING A SLOW; SHALLOW TURN TO L; TO CLR TFC EVEN MORE. AFTER TURN; WE WERE APPROX 30 DEGS L CORRECTION FROM ORIGINAL COURSE; AND RECEIVED CLRNC TO CONTINUE DSCNT TO FL330 WITH A DIRECT OBH VOR OUT OF FL350. WHEN ESTABLISHED AT FL330 I REQUESTED A DISCREET FREQ TO EXPLAIN WHY I VACATED FL390. A PHONE CALL WAS PLACED TO ZDV UPON ARR AT DEST. AN INACCURATE BAG COUNT; PLUS USING AN ESTIMATED WT VERSUS ACTUAL WT OF BAGS WAS DETERMINED TO BE THE MAIN PROB. SINCE WE LOADED ERRONEOUS DATA INTO THE FMC (WTS); IT CORRESPONDINGLY RETURNED TO US EQUALLY BAD INFO.