MD11 INITIATES UNCOMMANDED DSCNT WHILE AT CRUISE AT INTL RVSM AIRSPACE. FLT CREW COMPLIES WITH TCAS RA IN RESPONSE TO OPPOSITE DIRECTION TFC 1000 FT BELOW.

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OUR ACFT BEGAN AN UNCOMMANDED DSCNT IN MODERATE TO SEVERE TURB WHILE CRUISING S ON B576 AT FL380 APCHING SALMI WAYPOINT. AT THE SAME TIME AN ACFT WAS APCHING SALMI FROM THE S AT FL370. THE FO WHOSE LEG IT WAS; HAD BEEN MONITORING THE MERGE WITH THIS ACFT ON TCAS PRIOR TO THE EVENT. DESPITE IMMEDIATELY RECOGNIZING THAT OUR ACFT WAS STARTING TO DSND AND DISENGAGING THE AUTOPLT; WE HAD ALREADY ACHIEVED A RATE OF DSCNT OF ABOUT 1000 FPM. WITHIN SECONDS WE RECEIVED AN RA FROM TCAS AND FLT PATH GUIDANCE COMMANDING A CLB TO AVOID THE CONVERGING TFC. THE FO DID AN EXCELLENT JOB OF COMPLYING WITH TCAS BUT NOT BEFORE LOSING APPROX 300 FT OF ALT BEFORE CLBING. I WAS ABLE TO ACQUIRE THE TFC VISUALLY AND DETERMINED THAT WE WERE NOT IN IMMINENT DANGER OF A COLLISION; AND ESTIMATE THAT WE WERE ALREADY IN A POSITIVE RATE OF CLB BEFORE THE MERGE. WE CONTINUED THE CLB THROUGH FL380 STILL FOLLOWING TCAS GUIDANCE COMMANDS AND EVENTUALLY LEVELED OFF 340 FT HIGH. WE THEN RETURNED TO FL380 WITHOUT FURTHER INCIDENT AND RE-ENGAGED THE AUTOPLT. SHORTLY AFTER PASSAGE; THE OTHER ACFT RPTED THAT THEY HAD ALSO RECEIVED AN RA AND RPTED DEVIATING FROM THEIR ASSIGNED ALT IN COMPLIANCE WITH TCAS COMMANDS. ATC RESPONDED BY ASKING IF THERE WERE ANY FURTHER PROBS TO RPT. THE OTHER ACFT SAID NO; AND CONTINUED ENRTE. WE PROCEEDED TO OUR DEST UNEVENTFULLY. IT IS MY BELIEF THAT GIVEN THIS SET OF CIRCUMSTANCES THIS EVENT COULD NOT HAVE BEEN FORESEEN AHEAD OF TIME NOR COULD THE PF REACTED ANY MORE QUICKLY OR APPROPRIATELY. WE WERE BOTH RESTED; ALERT; AND AWARE OF THE APCHING ACFT; AND YET WITHIN SECONDS WE WERE RUDELY REMINDED THAT IN AN RVSM ENVIRONMENT THE REMARKABLE PRECISION IN TIME AND SPACE THAT OUR CURRENT TECHNOLOGY CAN PLACE US; MAKES ANY DEV A POTENTIAL DISASTER.