ALTDEV ALT OVERSHOT -- THE ALT CAPTURE MODE OF THE AUTOPLT MALFUNCTIONED AND THE ACFT DSNDED BELOW ITS ASSIGNED ALT.

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IFR FROM IKK TO BFF; I DECIDED A FUEL STOP WAS IN ORDER AND SELECTED N PLATTE; NE; LBF. ON DSCNT FROM 12000 FT TO 7000 FT I INADVERTENTLY DSNDED THROUGH MY ASSIGNED ALT; 7000 FT BY 320 FT. THIS CAUSED A CONFLICT ON ZDV RADAR BECAUSE OF SEPARATION OF LESS THAN 1000 FT. I IMMEDIATELY RECOGNIZED THE ERROR AND WENT BACK TO 7000 FT. THE EVENT WAS ONLY A FEW SECONDS IN LENGTH. DURING THIS; THE CTLR ASKED MY ALT AND I SAID THAT I WAS LOW AND WAS CORRECTING. HE GAVE ME A NUMBER TO CALL WHEN I LANDED WHICH I DID. MY ACFT; A PA60 IS EQUIPPED WITH KFC200 WITH ALT PRESELECT. I HAD TAKEN IT OFF ALT HOLD; SELECTED 7000 FT AND ARMED THE SYS. AT 7000 FT THE PRESELECT FAILED TO ARREST THE DSCNT AS EXPECTED. I IMMEDIATELY INTERRUPTED THE SYS AND HAND FLEW THE ACFT BACK TO 7000 FT. IT TOOK THE 320 FT TO ARREST THE DSCNT AND CORRECT IT. MY ACFT WAS FRESH OUT OF THE AVIONICS SHOP JUST 3 DAYS BEFORE THIS TRIP. REPAIRS WERE MADE TO THE AUTOPLT AND PRESELECT SYS. APPARENTLY SOME MALFUNCTION STILL EXISTS WHICH I WILL HAVE REPAIRED AS SOON AS I RETURN HOME. UNTIL THAT TIME I WILL HAND FLY THE CLB AND DSCNT MODES SO THIS PROB WON'T BE REPEATED. IN THE FUTURE; I WILL NEVER RELY ON THE PRE-SELECT EQUIP TO FUNCTION PROPERLY. THIS WHOLE THING IS EMBARRASSING FOR ME AS I CONSIDER MYSELF TO BE A GOOD PLT WITH OVER 2900 HRS. I FILE IFR EVEN IN GOOD VFR WX AS IT WAS THIS DAY FOR THE ADDED MARGIN OF SAFETY IFR OFFERS. I BELIEVE IN THE SYS. I REGRET HAVING HAD TO MAKE THAT PHONE CALL TO CTR BUT I WILL BE A BETTER PLT BECAUSE THIS HAPPENED; THIS I AM SURE OF. THE CTLR STATED THAT HE SAW A DSCNT TO 6500 FT. THIS DID NOT COINCIDE WITH MY ALTIMETER BECAUSE I ARRESTED THE DSCNT AT 6680 MSL AND CLBED BACK TO 7000 FT MSL IMMEDIATELY. AS FOR ACFT #2; HE WAS AT 6000 FT AND A FEW MI BEHIND ME AT MY 7 O'CLOCK POS. THE OTHER ACFT INVOLVED IN THIS ALT CONFLICT HAD RPTED MY ACFT IN SIGHT.