B767 CREW DEPARTED ASSIGNED ALT WHILE ATTEMPTING TO RESOLVE AN FMC PROB.

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IN CRUISE; AS I (PLT MONITORING) ENTERED REF +5 KTS ON RWY 29 AT OAK WAYPOINT; BOTH FMC'S WENT BLANK WHEN I PUSHED EXECUTE. LINE DRAWN THROUGH AUTOPLT (AND VNAV BUT NOT CERTAIN) ON ADI (AUTOPLT REMAINED ENGAGED); FMC'S SAID 'PERFORMANCE DATA NOT AVAILABLE' (OR SIMILAR TEXT). I TOLD FO TO WATCH THE AIRPLANE (HIS LEG) AND I WOULD SET UP RAW DATA TO STAY ON THE AIRWAY. I SET UP THE AIRWAY FOR FO AND THEN SAID I WOULD TRY TO RE-ENTER RTE/'INIT'/ETC. I THEN HEARD ALT ALERT; LOOKED UP AND SAW WE WERE 200-250 FT LOW AND DSNDING. AT THAT TIME; CTR SAID MODE C SHOWED US 300 FT LOW. FO HAD DISCONNECTED AUTOPLT AND BEGUN CLB TO ASSIGNED FL340 WHEN HE SAW DSCNT BUT THAT WASN'T TILL ALT ALERT WENT OFF. LEVELED OFF; RE-ENGAGED AUTOPLT; CONFIRMED ON AIRWAY AND I BEGAN RE-ENTERING RTE. EVENTUALLY ACCOMPLISHED. RESTORED VNAV AND LNAV. NO FURTHER MENTION FROM CTR. FMC'S WORKED NORMALLY REMAINDER OF FLT TO OAK. I WAS HEADS DOWN ENTERING DATA AND FO HAD WANDERED OFF TASK AS I DID IT. WITH THE MALFUNCTION OF THE FMCS; I SHOULD HAVE BEEN MORE VIGILANT AND WARY OF FURTHER DEGRADATION IN ITS PERFORMANCE. SUPPLEMENTAL INFO FROM ACN 701863: WE WERE IN CRUISE FLT AND THE AUTOPLT WAS ENGAGED WHEN WE RECEIVED FMC MESSAGES THAT INFORMED US OF AN AUTOPLT MESSAGE AND VNAV UNAVAILABLE ON THE ADI. AT THAT MOMENT BOTH FMC'S WENT COMPLETELY BLANK VIEWING NO INFO TO BACK UP OUR FILED FLT PLAN. BEING THE PF; I IMMEDIATELY VERIFIED HDG AND ALT TO MAKE SURE NO DEVS HAD OCCURRED. THE AUTOPLT WAS CHKED AND VERIFIED TO BE ENGAGED BY BOTH CAPT AND FO. THE CAPT BEGAN TO IDENT AND WORK ON RELOADING THE FMC WHEN THE AUTOPLT WENT INTO DEGRADED MODE. THERE WAS A LINE DRAWN THROUGH THE VNAV FMA ON THE ADI AS WELL. AGAIN; I CHKED TO MAKE SURE THAT THE AUTOPLT WAS ENGAGED AND IT APPEARED TO BE WORKING DESPITE THE MESSAGE. I TOOK MY ATTN AWAY FROM THE FLT INSTS FOR ABOUT 5 SECONDS TO REACH FOR HIGH ALT CHART WHEN THE CTR CTLR QUERIED US ABOUT OUR ALT AND I IMMEDIATELY LOOKED AT MY INSTS AND THE AUTOPLT WAS ENGAGED AND FLYING AWAY FROM THE ALT IN THE WINDOW. I IMMEDIATELY DISENGAGED THE AUTOPLT AND REGAINED THE APPROX 280 FT DEV BACK TO FL340. ATC DID NOT QUERY US ANY FURTHER ONCE WE TOLD THEM WE HAD AN AUTOPLT MALFUNCTION. WE RE-ENGAGED THE AUTOPLT AND THE CAPT BEGAN TO RELOAD THE FLT PLAN IN THE FMC WHILE I WAS USING RAW DATA TO NAV. ONCE THE FLT PLAN WAS RE-ENTERED; THE FMC TOOK THE FLT PLAN AND ALL APPEARED NORMAL WITH NO FURTHER PROBS. THE AUTOPLT WORKED NORMAL THE REST OF FLT AND WE EXPERIENCED NO DEVS OR PROBS.