ACFT EQUIP PROB -- THE SPDBRAKES AUTO-DEPLOYED DURING CRUISE ON THIS ACR AND IT LOST 1500 FT IN THE RECOVERY.

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IN CRUISE AT FL330; IN AND OUT OF CLOUD TOPS IN OCCASIONAL LIGHT TURB; OUR 757 PITCHED NOSE DOWN AND BEGAN A DSCNT. BOTH THE COPLT AND I GRABBED THE YOKE. THE ALT ALERT WARNING WENT OFF (300 FT BELOW 330). I DISCONNECTED THE AUTOPLT. THE PLANE WAS OUT OF TRIM NOSE DOWN. I SAW THE SPDBRAKE LEVER EXTENDED 3/4 FULL; CYCLING BTWN HALF AND 3/4 EXTENSION. I GRABBED THE LEVER AND PUSHED TOWARD RETRACT. THERE WAS RESISTANCE. IT WANTED TO EXTEND. FINALLY THE LEVER STAYED STOWED AND AIRSPD INCREASED MOMENTARILY TO HIGH SPD BUFFET; BUT NO OVERSPD OCCURRED. AT FL315 WE REGAINED FULL CTL WITH ELECTRIC YOKE TRIM AND BEGAN CLB BACK TO FL330. WE ADVISED ATC OF INCIDENT. AT FL330 WE TRIED TO ASSESS THE PROB. NO LOGIC FOR THE INCIDENT WAS REASONABLE; BUT IT ALL OCCURRED IN RAPID SUCCESSION. WE NOTIFIED ACR SYS MAINT BY RADIO AND THEY HAD NO HISTORY OF A SIMILAR PROB ON THE ACFT. THEY NOTIFIED OUR MAINT AT OUR DEST. FLT ATTENDANTS NOTED NO INJURIES TO ANYONE; BUT FOOD AND DRINKS WERE SPILLED -- PROBABLY AT ONSET OF DSCNT. AT DEST PLANE WAS HAND FLOWN TO MANUAL SPDBRAKE LNDG TO AVOID ANY RECURRENCE OF AUTO EXTENSION AT LOW ALT. EXITING RWY; SPDBRAKE LEVER WAS MANUALLY RETRACTED AND MADE A NOISY THUMP AS IT PASSED THE AUTO ARMED DETENT. I SHOWED THIS TO THE MECH AT THE GATE. UNTIL THE INCIDENT; THE FLT WAS NORMAL. THERE WAS NO TKOF WARNING; AND SPDBRAKES HAD NOT BEEN USED TO THAT POINT. CREW RESPONSE WAS RAPID; AND COPLT'S ASSISTANCE WAS VERY HELPFUL. I HAVE NO LOGICAL EXPLANATION FOR THIS PROB; SINCE SYS ARE DESIGNED TO PREVENT SUCH AN OCCURRENCE. WE PULLED THE AUTO SPDBRAKE CIRCUIT BREAKER AND GOT AN AMBER AUTO SPDBRAKE INOP WARNING (NORMAL). RESETTING THE CIRCUIT BREAKER; THE WARNING WENT OUT (NORMAL). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT SAID THAT MAINT CHANGED PART OF THE SPDBRAKE MECHANISM; BUT NOBODY SEEMS TO KNOW EXACTLY WHAT CAUSED THE INITIAL PART OF THE PROB. HE NOW KNOWS THAT THE AUTOPLT WAS TRIMMING NOSE DOWN TO COUNTERACT THE NORMAL PITCHUP MOMENT OF THE SPDBRAKE DEPLOYMENT; BUT SOME PEOPLE SPECULATED THAT SINCE THE SPDBRAKES ARE FLY- BY-WIRE PERHAPS ELECTRO MAGNETIC INTERFERENCE CAUSED DEPLOYMENT. HE FEELS THAT THE RECOVERY WAS AIDED BY THE USE OF ELECTRIC TRIM AS WELL AS FORCING THE SPDBRAKES DOWN. THE CAPT SAID THAT THIS IS THE FIRST UNUSUAL EVENT THAT HE HAS HAD WITH THE B757-200 AND SINCE THIS OCCURRENCE HE HAS FLOWN THIS ACFT SEVERAL TIMES. HE DOES NOT THINK THAT THERE WAS ANY WAKE TURB ASSOCIATED WITH THIS EVENT. CALLBACK CONVERSATION WITH RPTR ACN 308117 REVEALED THE FOLLOWING INFO: THIS CALLBACK WAS IN CONJUCTION WITH MULTI-ENGINE TURBOJET INCIDENT CALLBACK QUESTIONNAIRE. THE AUTOPLT DISCONNECTED AUTOMATICALLY BECAUSE OF PROTECTIVE ACFT SYSTEMS. THE RECOVERY MANEUVER WAS SMOOTH AND THE EVENT NOT NOTICED BY PAX. RPTR SUBSCRIBES HIGHLY TO UNUSUAL ATTITUDE TRNG.