AIR TAXI CARGO PLT HAS ALTDEV ON CLBOUT. ALT ALERTER IS SET CORRECTLY.

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I WAS CLBING OUT OF AGC ENRTE TO RIC. I WAS TALKING WITH PITTSBURGH APCH AND WAS CLRED TO CLB AND MAINTAIN 7000 FT. FOR SOME REASON I LEVELED OFF AT 5000 FT AND WAS THERE FOR SEVERAL MINS BEFORE HEARING FROM ATC. THEY QUESTIONED WHETHER I HAD IN FACT LEVELED OFF AT 5000 FT; AND ADVISED THAT I WAS TOLD TO CLB AND MAINTAIN 7000 FT. IN A MOMENT OF DISBELIEF I GLANCED AT MY MAKE-SHIFT ALT ALERTER AND WAS SURPRISED TO SEE THAT I HAD IN FACT SET 7000 FT. AFTER ACKNOWLEDGING MY MISTAKE AND OFFERING TO HURRY UP TO 7000 FT I WAS TOLD THAT I COULD REMAIN AT 5000 FT IF I PREFERRED BECAUSE THERE WAS APPARENTLY NO CONFLICT. I CHOSE TO REMAIN AT 5000 FT WITH NO FURTHER DISCUSSION. I FIND IT DIFFICULT TO COME UP WITH ANY REASONABLE EXPLANATION FOR MY OVERLOOK OF THE PROPER ALT. PERHAPS I WAS TIRED AND/OR STRESSED WITHOUT KNOWING; AND MY TEMPORARY ABSENT-MINDEDNESS WAS A RESULT. I WAS CERTAINLY AWARE OF MY INITIAL CLRNC; AS EVIDENCED BY MY SETTING OF 7000 FT INTO MY 'ALT ALERTER.' PERHAPS MY CLRNC OF 5000 FT THE PREVIOUS NIGHT ON THE SAME RUN AFFECTED THE SIT; ALTHOUGH IT CERTAINLY SHOULD NOT HAVE. IN A SINGLE PLT OP SUCH AS I WAS FLYING; ONE DOES NOT HAVE THE LUXURY OF THE SECOND SET OF EYES AND EARS TO HELP OUT IN THESE SITS. A SECOND PLT COULD HAVE PREVENTED MY PREMATURE LEVELING OFF AND WOULD CERTAINLY PREVENT THE RECURRENCE OF MANY SIMILAR SITS.