ALTDEV ALT EXCURSION WHEN ACFT AUTOPLT ALT HOLD FAILS. ACFT STARTS DSCNT TO OCCUPIED ALT. NMAC.

Narrative

ON A IFR FLT PLAN FROM GJT AND SDL; I WAS CLRED TO DSND FROM FL200 TO 16000 FT. UPON DSCNT THROUGH 16500 FT I CALLED CTR; ASKING FOR 14000 FT. THEY DENIED THE REQUEST STATING I HAD TFC; A KING AIR AT 2 O'CLOCK AT 15000 FT. AS SOON AS THEY STARTED HIM DOWN THEY WOULD GIVE ME LOWER. I LOOKED FOR AND RPTED HAVING THE TFC IN SIGHT. MY ACFT WAS ON THE AUTOPLT WITH THE ALT PRESELECT SET TO 16000 FT. I HEARD THE AUDIBLE AS IT APCHED 16000 FT AND FELT THE ACFT BEGIN TO LEVEL OFF. I SAW THE ALT CAPTURE LIGHT UP ON THE ALT PRESELECT. APPROX 1-2 MINS LATER AS I SCANNED THROUGH THE GAUGES. I NOTICED A GRADUAL DSCNT AND THE ALTIMETER SHOWED 15700 FT. I DISCONNECTED THE AUTOPLT AND BEGAN A CLB BACK TO 16000 FT. AS I STARTED MY CLB; ZAB CALLED TO VERIFY MY ALT. I RESPONDED THAT I WAS CLBING BACK TO 16000 FT AND THE AUTOPLT HAD DROPPED THE ALT HOLD. AFTER REACHING 16000 FT ABQ CALLED; CLRING ME TO DSND TO 11000 FT; 'IF I HAD THE KING AIR IN SIGHT AT 11 O'CLOCK POS.' I RESPONDED AND BEGAN A DSCNT TO 11000 FT USING THE AUTOPLT AGAIN. UPON REACHING 11000 FT THE AUTOPLT BEHAVED THE SAME AS IT HAD AT 16000 FT. I THEN TURNED THE AUTOPLT OFF AND HAND FLEW THE ACFT; MARKING THE AUTOPLT INOP. THE ACFT IS CURRENTLY IN MAINT HAVING THE AUTOPLT DIAGNOSED.