A G5 FLC OVERSHOOTS THEIR ASSIGNED ALT ON LEVELOFF WHEN THE FLT GUIDANCE COMPUTER FAILS TO AUTO CAPTURE THE ALT W OF TEB; NJ.

Narrative

AFTER DEPARTING TEB; ATC CLRED US TO 10000 FT ON AN ASSIGNED HEADING. AUTOPLT AND AUTOTHROTTLES WERE ENGAGED; VERT SPD OF 2000 FPM WAS COMMANDED. I NOTICED THAT THE ACFT WAS NOT FOLLOWING THE SPD COMMAND; WE WERE APPROX 230 KTS RATHER THAN 250 KTS AS PROGRAMMED. I DISENGAGED THE AUTOPLT AND FLEW THE COMMAND BARS TO ESTABLISH TO NORMAL PROFILE. WHEN THE PARAMETERS WERE MET; AUTOPLT WAS RE-ENGAGED AND SEEMED TO PERFORM NORMALLY. APCHING 10000 FT THE ACFT STARTED TO PITCH UP; TRIM RAN TOWARD FULL NOSE UP AND THE 'ELEVATOR MISTRIM NOSE UP' EICAS MESSAGE APPEARED. I PUSHED TCS TO OVERRIDE THE AUTOPLT BUT WE PASSED 10000 FT. AT THAT POINT THE PWR INCREASED TO FULL CLB PWR AS THE PROGRAMMED AUTOSPD INCREASED TO 300 KTS FROM 250 KTS. THIS INCREASED THE PITCH UP MOMENT. AUTOTHROTTLES AND AUTOPLT WERE BOTH DISENGAGED; BUT THERE WAS AN ALT EXCURSION OF 500 FT BEFORE I WAS ABLE TO RETURN TO 10000 FT. ATC NOTICED THE ALT AND HAD US CONFIRM WE WERE CLRED TO 10000 FT. IT WOULD HAVE REQUIRED A VIOLENT NEGATIVE 'G' FORCE TO HAVE STOPPED THE CLB AT 10000 FT. WE WERE IN VMC CONDITIONS AND HAD NO TCASII TFC NEAR OUR ALT. FGC (FLT GUIDANCE COMPUTER) #2 WAS SELECTED AND NO FURTHER PROBS WERE ENCOUNTERED. ON THE PREVIOUS FLT OF THIS ACFT; THE CREW HAD SQUAWKED THE FGC #1. MAINT WROTE OFF THE SQUAWK AS 'COULD NOT DUPLICATE #1 IAC (INTEGRATED AVIONICS CTL) OPS CHKS GOOD.' AFTER THIS FLT THE #1 IAC WAS REPLACED.