A C90 FLT DIRECTOR COMPUTER MALFUNCTIONED CAUSING A FAILURE TO CAPTURE THE ASSIGNED ALT AND AN 800 FT ALTDEV.

Narrative

I RECEIVED A XING CLRNC TO CROSS OCF AT 11000 FT. BEFORE REACHING 11000 FT; I WAS CLRED DOWN TO 9000 FT. I SET 9000 FT IN THE ALT PRESELECT AND VERIFIED THAT THE AUTOPLT WAS ARMED. I NOTICED THE PLANE NOT LEVELING OFF AND TOOK CTL AWAY FROM THE AUTOPLT AROUND 8800 FT (OR AT LEAST I THOUGHT). ME PULLING UP WAS COUNTERACTED BY A NOSE DOWN TRIM INPUT. I DISCONNECTED THE ELECTRIC TRIM SWITCH WITH NO CHANGE IN CTL. AROUND 8500 FT; ATC TOLD ME THAT I RECEIVED A CLRNC FOR 9000 FT. I REPLIED THAT I THOUGHT I HAD A RUNAWAY TRIM; DUE TO THE FACT THAT THE TRIM CONTINUED TRIMMING NOSE DOWN AFTER I HAD TURNED IT OFF. THEY GAVE ME A HDG TO STEER AWAY FROM TFC. I REACHED FOR THE AUTOPLT DISCONNECT SWITCH ON THE CTR CONSOLE (THIS WOULD BE THE SECOND DISCONNECT). BEFORE ACTUALLY PRESSING IT; THE AUTOPLT LET GO. TOTAL LOSS OF ALT WAS MAYBE 800 FT. IT WAS HARD TO SAY DUE TO THE PORPOISING OF THE AIRPLANE DUE TO FIGHTING THE AUTOPLT. SINCE THIS INCIDENT; THE PLANE HAS BEEN AT THE AVIONICS SHOP. WE RAN SOME TESTS AND FOUND A PROB IN THE COMPUTER OF THE FLT DIRECTOR. IT IS SHIPPED OFF FOR REPAIR. I HAD A PLANE WITH 4 KIDS AND 2 ADULTS ON THEIR WAY TO VACATION. NEEDLESS TO SAY; THE KIDS WERE EXCITED (NOT MY KIDS). I DON'T THINK THIS WAS A FACTOR; BUT MAYBE I COULD HAVE CAUGHT IT EARLIER; IF THERE WAS LESS DISTR.