AN AC690B FAILED TO LEVEL OFF AT ASSIGNED ALT OF 12000 FT.

Narrative

I HAD JUST LEVELED OFF AT 12000 FT AS I ENCOUNTERED ICING (LIGHT) CONDITIONS. THE CLB WAS BEING CONDUCTED WITH AUTOPLT ENGAGED. AT 11500 FT; I DECREASED RATE OF CLB TO 500 FPM AND ENGAGED ALT HOLD AT 12000 FT. I THEN PLACED MY ATTN ON THE OVERHEAD SWITCH PANEL TO SELECT PROP DEICE AND WINDSHIELD HEAT AND TO VERIFY THAT ENG IGNITION WAS ON. (ENG ANTI-ICE IS ANNUNCIATED ON A LOWER ANNUNCIATOR PANEL AND WAS OBSERVED TO BE ACTIVATED.) AT THAT MOMENT; THE ALT ALERTER GAVE AN AURAL ALERT THAT TARGET ALT WAS EXCEEDED AND I LOOKED AT FLT INSTS TO FIND THE ACFT IN AN UNCOMMANDED NOSE HIGH; WINGS LEVEL CLB AT 1100 FPM. I IMMEDIATELY TOOK MANUAL CTL OF THE YOKE; PITCHING DOWN AND SIMULTANEOUSLY DISENGAGING THE AUTOPLT. I RECOVERED AT 12400 FT AS ATC ALERTED ME TO TFC IN MY VICINITY. I TRIMMED THE ACFT AND CAUTIOUSLY RE-ENGAGED THE AUTOPLT WHICH FUNCTIONED CORRECTLY FOR THE DURATION OF THE FLT. I BELIEVE THIS OCCURRED BECAUSE OF THE MULTI TASKING INVOLVED WITH SIMULTANEOUS LEVELING OFF AND ENCOUNTER WITH ICING CONDITIONS. THE ALT ALERTER SERVED ITS PURPOSE AND THIS WAS A NEVER BEFORE OBSERVED AUTOPLT MALFUNCTION IN THIS PARTICULAR ACFT.