C208 PLT HAD A MODE C READOUT THAT DIFFERED FROM THE ACFT ALTIMETER.

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2ND ALT INCURSION IN 4 DAYS. ON THIS DAY THE AUTOPLT WAS SHOWING 100 FT TOO HIGH. ATC SAID HE WAS SHOWING WE WERE AT 7500 FT. ALTIMETER WAS ONLY OFF THE SETTING OF .3 (30.06). I HAD MAINT TAKE A LOOK UPON MY RETURN. ATC SHOWED ME AT 7500 FT 2 MINS AFTER I RESET THE ALTIMETER; AND DSNDED 100 FT. ATC HAS SAID NOTHING ON BOTH OCCASIONS; HOWEVER; THEY ARE USED TO ME; AND ME OF THEM. I AM CONCERNED WITH MY MENTAL ERRORS THIS WEEK AND I WONDER IF THEY ARE SURPRISED AT ME. WE HAVE WORKED TOGETHER DURING THE HURRICANE SEASON WITH NO PROBS. CONTRIBUTING FACTORS: MAYBE THE COMPLACENCY OF EXCELLENT VFR WX; COMBINED WITH CHANGING FROM 2 PLT CREWS; TO SINGLE PLT OPS. BOTH SITS ARE SIMILAR; AND I FEEL IT'S MORE MY MENTAL ERROR; THAN MECHANICAL. CORRECTIVE ACTIONS: WHEN SWITCHING FROM 2 PLT TO SINGLE PLT -- NOW; INSTEAD OF JUST MENTALLY 'NOTING' THE ALT LIMIT; I HAVE STARTED USING A POST-IT NOTE EACH TIME A CHANGE IS MADE; CONCERNING ALT CHANGES. ALSO; I HAVE TOLD PAX EACH AND EVERY TIME ABOUT THE 'CRITICAL PHASE OF FLT.' SOMETIMES I USED TO DO IN THE PAST AND APPARENTLY FELL SHORT A FEW TIMES. ALSO; A PLT MEMO WAS POSTED CONCERNING MY ERRORS; AND ASKED IF ANYONE HAS HAD SIMILAR PROBS. SUGGESTIONS FROM THE PLT CREWS ARE NOW BEING ACCEPTED. AN 'ALT ALERT BOX' WOULD BE NICE IN A SINGLE OR MULTI-ENG OP; HOWEVER; OWNERS SEE THE MONEY SIDE OF ITS IMPLEMENTATION.