B777 FO REPORTS AUTOPILOT MALFUNCTION AS CAPT AND RELIEF PILOT SWITCH POSITIONS DURING DESCENT; CAUSING ALT AND COURSE DEV WITH PAX INJURIES.

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I HAD JUST RETURNED TO THE FLT DECK FROM MY BREAK; WHICH WAS THE LAST BREAK PERIOD. THE RELIEF PLT HAD LEFT THE COCKPIT AND THE CAPT WAS GOING TO TAKE A QUICK BREAK UPON HIS RETURN. WE WERE ALREADY IN THE INITIAL DSCNT WHICH HAD BEEN INITIATED BY THE CAPT. AS I RECALL; WE HAD BEEN CLRED TO FL360. I WAS INVOLVED WITH FINDING THE ARR PAGES AND APCH CHARTS AND WAS NOT PAYING ATTN TO THE PROGRESS OF THE DSCNT. I NOTICED THE ACFT MAKING A SIGNIFICANT ROLL TO THE R AND LOOKED TO THE MAP AND DETERMINED WE WERE DEVIATING SIGNIFICANTLY TO THE R OF THE ARR PATH. AT THE SAME TIME THE CAPT HAD GOTTEN UP TO LET THE RELIEF PLT BACK IN. I SAID SOMETHING TO THE EFFECT OF 'WHY ARE WE TURNING OFF COURSE?' I NOTED THAT LNAV WAS ENGAGED AND I DISCONNECTED THE AUTOPLT AND BEGAN TO CORRECT BACK TO THE COURSE. SIMULTANEOUSLY; THE CAPT SAID 'WATCH YOUR ALT.' I CHKED THE ALT AND WE WERE ABOUT 400 FT LOW FROM FL360. I INITIATED A CLB AS WE WERE STILL DSNDING AND CLBED BACK TO FL360. I WAS ATTEMPTING TO MAKE THE CORRECTION AS SMOOTH AS POSSIBLE BUT ALSO AS EXPEDITIOUS AS POSSIBLE WHILE TRYING TO PULL AS FEW G'S AS POSSIBLE. AS WE STARTED TO LEVEL OFF AT FL360 CTR CALLED AND QUESTIONED OUR ALT. BY THIS TIME THE RELIEF PLT HAD SAT DOWN IN THE CAPT'S SEAT AND RESPONDED LEVEL AT FL360. CTR SAID THEIR RADAR SHOWED US 400 FT LOW. I RESPONDED TO CTR THAT WE HAD EXPERIENCED AN AUTOPLT MALFUNCTION AND WERE CORRECTING THE DEV. THEIR RESPONSE WAS; 'THAT IS GOOD INFO TO KNOW'? OR WORDS TO THAT EFFECT. WHEN I NOTED THE FLT PATH DEV AND ASSUMED AN AUTOPLT MALFUNCTION; I WAS (DURING THIS PROCESS) THINKING ABOUT A SIMILAR AUTOPLT MALFUNCTION I EXPERIENCED LAST MONTH. WE HAD A MALFUNCTION THAT REQUIRED A DIVERSION. THE CAPT WAS FLYING AND DURING THE TURN OFF COURSE; HDG SELECT WAS CHOSEN. EVERY TIME THE CAPT TURNED R; THE ACFT TURNED L. WE EXPERIENCED THIS SEVERAL TIMES BEFORE HE CHOSE TO HAND FLY THE ACFT. THE MALFUNCTION WAS WRITTEN UP. AS WE PROGRESSED IN THE ARR; THE PURSER ADVISED US THERE WERE 5 INJURIES RPTED DUE TO THE FLT PATH CORRECTION. I BELIEVE THEY SAID 3 PAX AND 2 FLT ATTENDANTS WERE INJURED. THE CAPT ADVISED THE PURSER THAT THE DEV HAD LIKELY BEEN CAUSED BY A WAKE TURB ENCOUNTER. COORD OF THIS ISSUE TOOK PLACE BTWN THE CAPT AND THE PURSER AND I WAS NOT SURE OF ANY SPECIFIC DETAILS OF INJURIES BUT IT SEEMED TO BE THAT ULTIMATELY THE INJURIES WERE SUPERFICIAL. I WAS NOT AWARE OF ANY FURTHER ACTION IN THIS REGARD. HUMAN FACTORS INVOLVED WERE THAT THERE WAS A FAIR AMOUNT OF CONFUSION IN THE COCKPIT INVOLVED IN THE CREW CHANGEOVER PROCESS AND THE DEV/MALFUNCTION COULD NOT HAVE OCCURRED AT A MORE VULNERABLE POINT. SUPPLEMENTAL INFO FROM ACN 787405: DURING INITIAL STAGES OF DSCNT WE WERE CLRED TO DSND FROM FL390 TO FL360. FO WAS FLYING WITH AUTOPLT ENGAGED AND IN LNAV AND VNAV. AS WE WERE DSNDING THROUGH FL365 THE ACFT MADE A RAPID BANK OF 45 DEGS TO THE R OFF COURSE IN APPROX 15 SECONDS. THERE WAS NO MAP SHIFT OCCURRING; UPDATING WAS ASSURED AND I IMMEDIATELY CHKED FMA'S TO CONFIRM MODE OF OP; VERIFYING THAT LNAV AND VNAV WERE ENGAGED. THE FO VOICED CONCERN AND CONFUSION AS TO WHAT WAS HAPPENING. CONCURRENTLY; THE ACFT DSNDED THROUGH THE CLRED FL360; AT WHICH TIME I YELLED 'ALT' AND 'DISENGAGE THE AUTOPLT AND FLY THE ACFT.' THE FO RESPONDED QUICKLY; AVOIDING THE NECESSITY OF TAKING OVER CTL OF THE ACFT. THE COMBINATION OF CORRECTING BACK L TO THE AIRWAY AND ARRESTING THE DSCNT RESULTED IN PULLING ABOUT 2 G'S. WE BOTTOMED OUT AT FL354 AND CORRECTED BACK TO THE CLRED FL360. THE ATC FACILITY NOTED A DEV OF 400 FT AND ASKED REASON FOR THE DEV; TO WHICH THE FO RESPONDED 'AN AUTOPLT MALFUNCTION.' THE ATC FACILITY ACKNOWLEDGED THE COM BUT DID NOT ASK FOR ANY OTHER INFO OR ACTIONS ON OUR PART. WE WERE INFORMED BY THE PURSER THAT 2 FLT ATTENDANTS AND 1 PAX WERE KNOCKED OFF THEIR FEET AND SUSTAINED SOME AMOUNT OF DISCOMFORT/INJURY. WHILE NO APPARENT CAUSES WERE OBVIOUS; THE 1 POSSIBLE CAUSE WAS A WAKE TURB ENCOUNTER; ALTHOUGH INCONCLUSIVE DUE TO NO TCAS VISUALS TO CONFIRM THAT CONCLUSION.