FLC DEV FROM ATC CLRNC IN SPD DEV. AUTO THROTTLE SYS FAILURE. CREW EXPERIENCED DIFFICULTY IN THE DISCONNECT OF THE AUTO THROTTLE SYS.

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I WAS THE FO ON FLT ATL-LGW. WE WERE CRUISING AT FL330 BTWN SPA AND GSO. IT WAS THE CAPT'S LEG AND HE WAS DOING THE REQUIRED NAV ACCURACY CHKS FOR AN OCEANIC XING. I WAS FILLING IN THE FLT LOG AND THE SO WAS WORKING ON THE FUEL CHART. THE L-1011-500 WE WERE FLYING HAD THE PERFORMANCE MGMNT SYS WHICH WAS IN USE; GIVING COMMANDS TO THE AUTO THROTTLE SYS INDICATING .83 MACH. APPARENTLY THE PERFORMANCE MGMNT SYS STARTED COMMANDING BAD INPUT TO THE AUTO THROTTLES. INSTEAD OF DISCONNECTING AND GIVING SOME KIND OF WARNING; THE AUTO THROTTLE SYS RESPONDED BY REDUCING THE THROTTLES TO IDLE THRUST. WITH NO WARNING OR INDICATIONS OF ANYTHING ABNORMAL; THE AIRSPD APPARENTLY DECAYED ABOUT 40 KTS LOW. OBSERVING THE LOW AIRSPD THE CAPT IMMEDIATELY SHOVED THE THROTTLES FORWARD AND CALLED FOR A LOWER ALT IN AN ATTEMPT TO REGAIN THE AIRSPD. I CALLED ATC AND ASKED FOR FL310 IMMEDIATELY. THE CAPT HAD SOME DIFFICULTY DISCONNECTING THE AUTO THROTTLE SYS AND HAD TO FIGHT THE THROTTLES MOMENTARILY. WE WERE GIVEN FL310 BY ATC AND REGAINED THE AIRSPD DURING THE DSCNT. AFTER EVERYTHING APPEARED TO BE WORKING NORMAL AGAIN; WE REQUESTED AND RECEIVED FL330 AND CLBED BACK UP WITHOUT INCIDENT. LATER INFLT THE PERFORMANCE MGMNT SYS AND AUTO THROTTLE SYS AGAIN MALFUNCTIONED AND WE DISCONTINUED THE USE OF THE AUTH THROTTLE SYS. THE APPROPRIATE LOGBOOK ENTRIES WERE RECORDED.