AN LR35 PIC OVERSHOOTS HIS AMENDED ALT WHEN THE ALTIMETER BECOMES ERRATIC AND UNRELIABLE 25 MI NW OF VNY; CA.

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DEPARTED VNY ON THE CANOGA 8 DEP. WHILE ON A 320 DEG VECTOR AND LEVEL AT 4000 FT SOCAL DEP SAID CLB TO 15000 FT. SHORTLY AFTER THAT THE CTLR CAME BACK AND RESTR US TO 8000 FT FOR TFC AT THE 10 O'CLOCK POS. AS I LEVELED OFF; THE CTLR SAID TO MAINTAIN 8000 FT; TFC AT 10 O'CLOCK POS. I TOLD THE COPLT I HAD TFC IN SIGHT; HE CONCURRED AND HESITATED TO CALL TFC TO THE CTLR DUE TO BEING OVERWHELMED. AS I REACHED 8000 FT; THE CTLR SAID TO MAINTAIN 8000 FT AND IMMEDIATELY CAME BACK AND SAID TO DSND TO 8000 FT. AS THE CTLR WAS TALKING I NOTICED THE ALTIMETER WAS FLAGGED OFF AND THEN FLICKED ON AND OFF IN A RAPID MANNER AND THEN WENT TO NORMAL (FLAG OFF) AND THE ALTIMETER RAPIDLY CLBED UP. ON RECOGNITION OF THIS I IMMEDIATELY BEGAN DSCNT TO 8000 FT. THE CTLR THEN INSTRUCTED US TO CLB TO 15000 FT AND THEN CHANGED US OVER TO ANOTHER FREQ AND THAT CTLR ASKED US TO CALL SOCAL ON THE PHONE FOR POSSIBLE ALTDEV. THIS PROB WITH THE ALTIMETER REPEATED ITSELF BRIEFLY WHILE DSNDING. UPON LNDG; I GROUNDED THE ACFT AND HAD MAINT WORK ON THE ACFT. THEY COULD NOT FIND THE PROB OR GET IT TO DUPLICATE ITSELF. THEY RELEASED THE ACFT. ON THE NEXT LEG; THE PROBS AROSE AGAIN; BUT ONLY FOR A 2-SECOND PERIOD (FLICKER) AND THEN RETURNED TO NORMAL OTHERWISE. I AGAIN GROUNDED THE ACFT AND AT SUS FOUND A PROB WITH THE SDC MODULE (STATIC DEFECT CORRECTION MODULE) AND REPLACED IT WITH AN OVERHAULED UNIT. THE PROB HAS NOT RESURFACED SINCE. CONTRIBUTING FACTORS: 1) PROB WITH ALTIMETER (STATIC DEFECT CORRECTION MODULE); 2) LOOKING FOR TFC; DIVERTING ATTN AWAY FROM INSTS AT CRITICAL TIME; 3) RAPID SEQUENCE OF EVENTS WITH INEXPERIENCED COPLT (2ND FLT IN A JET). HUMAN PERFORMANCE CONSIDERATIONS: 1) COPLT FAILED TO RECOGNIZE ANYTHING WAS WRONG. 2) COPLT FAILED TO RADIO TFC IN SIGHT WHEN INSTRUCTED TO.