FLC DEVIATES FROM ASSIGNED ALT.

Narrative

PAX FLT MCO-LGA; APCHING OTT UNDER CTR CTL AT FL310 WITH AUTOPLT ALT HOLD ENGAGED. WE WERE CONFIGURING THE RADIOS FOR THE NANCI ARR TO LGA; AND WHEN I LOOKED BACK AT MY ALTIMETER IT READ 400 FT HIGH (31400 FT). IMMEDIATELY CHKED THE ALT HOLD; WHICH WAS STILL ON; THEN DISENGAGED THE AUTOPLT; AND FELT THE ALT HOLD SWITCH CLICK OFF. AT THE SAME TIME; CTR CALLED TO ASK IF WE WERE LEVEL AT FL310. THE FO LOOKED AT HIS ALTIMETER; WHICH WAS READING WITHIN THE NORMAL DIFFERENCE FOR AN UNCORRECTED ALTIMETER (150 FT HIGH) AND REPLIED THAT WE WERE AT FL310. I INITIATED A SLIGHT DSCNT; AND TURNED 'OFF' THE CORRECTION TO MY ALTIMETER; AT WHICH TIME IT READ THE SAME AS THE FO'S ALTIMETER (PLUS 150 FT) AND LEVELED ON IT AT FL310. ON THE WAY INBOUND TO LGA WE ATTEMPTED TO TROUBLESHOOT THE PROB; BUT WHEN I TRIED TO APPLY CORRECTION TO MY ALTIMETER; IT BEGAN A SLOW CLB BY ITSELF. WE SUSPECT; BASED ON PRIOR EXPERIENCE; THAT THIS WAS A FAILURE OF THE CAPT'S AIR DATA COMPUTER WHICH; AMONG OTHER THINGS; CORRECTS THE CAPT'S ALTIMETER AND DETERMINES 'MODE C' ALT READOUT. WE WROTE IT UP AS SUCH UPON ARR IN LGA AND TURNED IT OVER TO MAINT FOR CORRECTION.