B757 CREW ENCOUNTERS MODERATE TURB AND DOWNDRAFT AT CRUISE ALT; LOSES ALT.

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DURING CLBOUT FROM MCO WE SAW SOME WX AHEAD ON THE RADAR AND VISUALLY. ATC HAD TOLD OTHERS OF LIGHT TO MODERATE CHOP; SO I CALLED THE FLT ATTENDANTS AND INFORMED THEM OF THE POTENTIAL AND THE TIME FRAME. THEN I MADE THE DEP PA TO INCLUDE THE POTENTIAL FOR A CHOPPY RIDE IN THE CLB. THE CLB WAS BASICALLY SMOOTH. WE LEVELED OFF AT FL360 AND RPTED LEVEL AS REQUESTED. WE WERE NOW IN THE LINE OF WX BUT THERE WAS NO RADAR RETURNS AROUND US OR IN FRONT OF US. THE RADAR RETURNS WERE QUITE A BIT N AND S OF OUR POS. WE WERE IN THE CLOUDS WITH ENG ANTI-ICE ON. ALMOST IMMEDIATELY UPON LEVELING OFF; WE RECEIVED LIGHT OCCASIONALLY MODERATE CHOP. THE ACFT STARTED LOSING ALT SLOWLY AND THE FO CALLED OUT THE DEV AS I REACHED FOR MAX CONTINUOUS THRUST AND PUSHED THE THROTTLES FORWARD MANUALLY. THE LOSS CONTINUED AS I RAISED THE PITCH ATTITUDE. WITH THE PWR AT MAX CONTINUOUS AND THE PITCH INCREASING; THE ALT WAS STILL DSNDING. FO CALLED ATC TO DECLARE THE UNCTLED ALT LOSS IN AN EXTREME DOWNDRAFT. THE CHOP WAS STILL NO WORSE THAn MODERATE. I INCREASED PITCH MORE BUT WE DSNDED TO FL355.5. ATC VECTORED ANOTHER ACFT AWAY FROM OUR POS AND CLRED US TO DSND TO FL340 AND DIRECT TO CEW. AT THAT TIME WE WERE THROUGH THE EVENT AND TOLD THEM WE WOULD CLB BACK TO FL360 AND PROCEED TO CEW. WE WERE NOT IN ANY VISUALLY NOTICEABLE CONVECTIVE ACTIVITY (RADAR OR OUTSIDE).