B767-300 ACFT ON A GAR AND LEVELOFF; PF; CAPT; DISCONNECTED AUTOPLT AND ACFT PITCHED UP GAINING 300 FT BEFORE FLC COULD RETURN TO ASSIGNED ALT.

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CONDUCTING ILS RWY 13R APCH TO BFI IN SEATTLE. AT THE LOM; CHKED IN WITH BFI TWR. WERE TOLD WE WERE #2 BEHIND ANOTHER ACFT; WE WOULD RECEIVE LNDG CLRNC FURTHER IN. AT ABOUT 500-700 FT; RECEIVED LNDG CLRNC AND ALSO THAT A CESSNA WOULD BE DEPARTING PRIOR TO OUR ARR. DUE TO SOME FOG WE WERE UNABLE TO SEE THE CESSNA UNTIL ABOUT 200 FT AGL. THE CESSNA HAD NOT BEGUN ITS TKOF ROLL. BFI TWR WAS YELLING AT THE CESSNA TO TKOF AND THAT WE WERE STILL CLRED TO LAND. WE HAD AN APCH REF SPD OF 145 KIAS AND CLOSING IN ON CESSNA SO ELECTED TO GAR. ON THE GAR WE WERE CLRED TO 6000 FT. THEN CLBING THROUGH 4000 FT WE WERE CLRED TO 5000 FT. THE FO AND AUTOPLT WAS ACTING STRANGE; TRYING TO PITCH ACFT EXCESSIVELY HIGH. AT 5000 FT WE LEVELED OFF AND I THEN BEGAN TO LOAD APCH BACK INTO FLT COMPUTER. AT THIS POINT THE CAPT DISCONNECTED AUTOPLT AND THE EXCESSIVE NOSE-UP TRIM CAUSED US TO CLB TO 5300 FT. I CALLED OUT THE ALTDEV AND THE CAPT CORRECTED BACK TO 5000 FT. AS THE PNF; ONCE YOU START LOADING THE COMPUTER; YOU'RE OUT OF THE LOOP. THE CAPT SHOULD HAVE ANNOUNCED THAT HE WAS DISCONNECTING THE AUTOPLT. THIS WOULD HAVE GIVEN ME NOTICE TO HELP HIM TO SCAN THE FLT INSTS. THE REST OF FLT AND NEXT APCH WERE NORMAL.