ACR FLC DID NOT MEET ALT XING AND SPD RESTR DUE TO MIS- PROGRAM OF FMS.

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ENRTE TO SFO WE HAD PROGRAMMED THE COMPUTER FOR MODESTO 2 ARR TO CROSS CEDES AT 250 KTS AND 11000 FT. WHEN ADDING THE FMS RWY 28R APCH WHILE STILL AT CRUISING ALT JUST PRIOR TO OUR COMPUTED DSCNT POINT; THE CEDES RESTR DROPPED OUT WITHOUT OUR NOTICING IT (WE WERE PREOCCUPIED WITH OTHER DUTIES AND THE VNAV PATH SHOWED RIGHT ON). I SUDDENLY NOTICED THE COMPUTER GLITCH AND COMMENCED A FULL DRAG DSCNT. ABOUT THIS TIME CTR ASKED US IF WE COULD STILL MAKE THE CEDES RESTR. WE SAID YES BUT WE MIGHT BE A LITTLE FAST. WHEN IT BECAME APPARENT TO US THAT WE COULD NOT MAKE THE RESTR WE TRIED TO ADVISE CTR BUT GOT NO ACKNOWLEDGEMENT. WE CROSSED CEDES AROUND 12000 FT AT 300 KTS. I KNOW ONE SHOULD ALWAYS BACK UP THE FMS BUT THERE ARE TIMES WHEN ONE BECOMES VERY BUSY IN A 2 PLT COCKPIT AND ONE RELIES ON THE COMPUTER. THIS TIME IT BIT US. SUPPLEMENTAL INFO FROM ACN 310000: WHEN ATC AGAIN SPECIFIED CEDES AT 11000 FT AND 250 KTS WE WERE DSNDING IN VNAV AND THE CAPT WAS MAKING THE PA. I RELAYED THE RESTR AGAIN BUT WAS FOCUSED ERRONEOUSLY ON THE FACT THAT IT WAS AFTER THE HR AND NO CURRENT ATIS.