ALTDEV ALT OVERSHOT BY THE FLT CREW OF A B737-800 DURING DSCNT AFTER EXPERIENCING AN 'INDICATED AIRSPD DISAGREEMENT' PROB WITH THEIR EFIS SYS ON FREQ WITH ZJX; FL.

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DURING CRUISE AT FL350 WE RECEIVED AN 'INDICATED AIRSPD DISAGREEMENT' LIGHT ON THE CDU. IT WAS NOTED AND VERIFIED BY XCHKING THE STANDBY AIRSPD AGAINST THE CAPT'S AND FO'S AIRSPD THAT THE CAPT'S AIRSPD WAS THE INCORRECT ONE (+/-5-10 KTS). THE CAPT ASSURED I WAS FLYING THE ACFT; AND THEN CALLED OUR MAINT CTL VIA COCKPIT AIR PHONE TO TROUBLESHOOT THE PROB. (WE HAD PREVIOUSLY ACCOMPLISHED THE 'INDICATED AIRSPD DISAGREEMENT' AND 'AIRSPD UNRELIABLE' CHKLISTS.) WHILE ON THE PHONE; WE BEGAN TO ENCOUNTER MODERATE TURB AT FL350. IT WAS DETERMINED THAT WE SHOULD TRY AND DSND TO A LOWER FLT LEVEL THAT WAS SMOOTHER. I CALLED ZJX AND WE WERE GIVEN CLRNC TO DSND TO FL290. I INITIATED THE DSCNT USING THE AUTOPLT IN LEVEL CHANGE MODE. WHILE DSNDING THROUGH APPROX FL330; I EXPERIENCED A LOSS OF THE FO'S AIRSPD INDICATOR AND ALTIMETER FOLLOWED BY A LOSS OF MY MAP DISPLAY AND A 'SINGLE FMC OP' ANNUNCIATION ON THE FMC. AT THIS POINT; I DISCONNECTED THE AUTOPLT AND BEGAN TO HAND FLY THE ACFT. I SHALLOWED THE DSCNT RATE AND TUNED PIE VOR INTO THE #2 VHF NAV AS A MANUAL BACKUP FOR THE CAPT'S MAP DISPLAY. THE CAPT SAW WHAT WAS OCCURRING AND ASKED IF I NEEDED HIS ASSISTANCE. THE CONDITIONS WERE VMC AND THE SIT WITH THE DSCNT AND NAV APPEARED STABILIZED; SO I TOLD HIM I WAS OK. HE CONTINUED TO TROUBLESHOOT OUR PROBS WITH MAINT AND I CONTINUED TO FLY THE ACFT. FOR THE DSCNT I WAS UTILIZING THE CAPT'S SIDE INST DISPLAY. I XCHKED THE STANDBY AIRSPD INDICATOR AGAINST THE CAPT'S AIRSPD INDICATOR TO ENSURE I WAS FLYING A SAFE AIRSPD. I WAS USING THE CAPT'S ALTIMETER FOR ALT REF; BECAUSE IT IS MORE EASILY READABLE THAN THE STANDBY DISPLAY. THERE WAS ALSO NO REASON TO THINK IT INACCURATE SINCE WE HAD XCHKED IT AT FL350 AGAINST THE STANDBY AND FO'S SIDE AND FOUND IT TO BE ALRIGHT. LEVELING AT FL290; I ACTUALLY OVERSHOT BY 100 FT TO FL289. ZJX CALLED AND TOLD US TO MAINTAIN FL290. I CORRECTED TO FL290. ZJX CALLED AGAIN AND TOLD US TO MAINTAIN FL290. AT THIS POINT; I THOUGHT SOMETHING MIGHT BE WRONG AND XCHKED THE CAPT'S ALTIMETER AGAINST THE STANDBY ALTIMETER AND FOUND THE STANDBY AT FL284. I THEN FLEW THE ACFT TO FL290 ON THE STANDBY ALTIMETER. BEFORE WE WERE HANDED TO THE NEXT CTLR WE VERIFIED THAT JAX SHOWED US AT FL290. THEY CONFIRMED THAT THEY DID AND WE INFORMED THEM OF OUR INSTRUMENTATION PROB. WE REPEATED THIS PROCESS WITH THE NEXT CTLR AS WELL. DURING THE REMAINDER OF THE FLT WE WERE ABLE TO RESTORE THE FO'S INST DISPLAYS AND RESTORE DUAL FMC OP. IF I WERE TO DO THIS OVER; I WOULD HAVE ASKED THE CAPT TO DISCONTINUE THE TROUBLESHOOTING CALL WITH MAINT; AND HAD HIM BACK ME UP. I ALSO WOULD HAVE DISREGARDED THE CAPT'S INST DISPLAY; AND FLOWN THE ACFT PURELY BY THE STANDBY AIRSPD INDICATOR AND STANDBY ALTIMETER.