SMT PLT OVERSHOOTS ALT ON CLB.

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DEPARTED ABY; CONTACTED ZJX; HDG ASSIGNED 310 DEGS TO 2000 FT INITIALLY; THEN TO 5000 FT. AFTER FLYING THIS HDG A FEW MINS; ZJX CLRED US TO NALIZ INTXN TO MAINTAIN 5000 FT. IT WAS COOL AND WE WERE CLBING APPROX 1800 FPM. AUTOPLT WAS ENGAGED AND ALT PRESELECT SET FOR 5000 FT. I DIVERTED MY ATTN BRIEFLY TO THE LORAN TO SET UP DIRECT COURSE TO NALIZ. I LOOKED BACK AND FOUND ACFT HAD GONE THROUGH 5000 FT TO 5800 FT AND THE AUTOPLT AND ALT PRESELECT WERE OFFLINE AND CLB HAD PASSED THROUGH ASSIGNED ALT. ZJX CTLR NOTED THE ERROR ABOUT THE SAME TIME AND CALLED REQUESTING MY ALT. THE ACFT HAD REACHED 6000 FT AS I WAS BEGINNING TO CORRECT AND RETRIM. THE CTLR STATED THAT I HAD ACKNOWLEDGED THE 5000 FT ASSIGNMENT TO CLB AND MAINTAIN WHICH IS CORRECT. HE THEN ASSIGNED 6000 FT AND SHORTLY THEREAFTER 7000 FT. THE CTLR ADVISED ME TO CALL AND TALK TO QUALITY ASSURANCE DIVISION OF FAA WHICH I DID UPON LNDG. I ADVISED ZJX WHEN ASKED TO CALL QUALITY ASSURANCE WHAT HAD CAUSED THE ALTDEV (AUTOPLT AND ALT PRESELECT FAILURE OR PERHAPS KNOCKED OFF LINE BY HITTING THE CTL WHEEL TRIM SWITCH WHEN I TURNED TO RESET THE LORAN). I ALSO RPTED THIS TO QUALITY ASSURANCE PERSONNEL BY PHONE. I CERTAINLY RECOGNIZE THE POTENTIAL DANGER OF THIS EQUIP FAILURE AND WILL QUICKEN MY INST SCAN IN THE FUTURE. ALSO PLAN TO HAVE THE AUTOPLT/ALT PRESELECT SYS CHKED.