FLC OF AN MLG CLBED ABOVE ASSIGNED ALT DUE TO THE INADVERTENT DISENGAGEMENT OF THE AUTOPLT HOLD. WHEN THE FLC NOTICED THAT THE ACFT WAS CLBING THEY IMMEDIATELY RETURNED TO ASSIGNED.

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BTWN OLM AND BTG WE WERE ON A DELAY VECTOR AND HAD BEEN SLOWED FROM MACH .77 TO MACH .72. ZSE TOLD US TO RESUME NORMAL SPD AND PROCEED TO OLM. THE CAPT; USING THE MCP; INCREASED ACFT SPD FROM MACH .72 TO MACH .77 AND STARTED A L TURN BACK TO OLM. THE THROTTLES ADVANCED NORMALLY AND THE CAPT REPROGRAMMED THE FMS FOR MACH .77 CRUISE AND MACH .77/300 FPM DSCNT SO HE COULD ENGAGE VNAV. WHILE HE WAS DOING THIS; THE AIRPLANE DROPPED OUT OF ALT HOLD AND ENGAGED IN CTL WHEEL STEERING PITCH AND BEGAN A VERY GRADUAL CLB WITH THE AUTOPLT ON. WHILE THIS WAS HAPPENING; I WAS REPROGRAMMING THE ARR AND RWY AT SEATAC (THEY WERE IN THE MIDDLE OF CHANGING FROM A N FLOW TO A S FLOW AND THERE WAS SOME UNCERTAINTY FOR A FEW MINS WHICH DIRECTION WE WERE GOING TO LAND). WHEN I WAS DONE WITH THE FMS; I LOOKED AT MY FLT INSTS AND NOTICED WE WERE CLBING ABOUT 400 FPM AND HAD CLBED ALMOST 800 FT. THE CAPT STOPPED THE CLB AND DSNDED BACK TO FL280 WHERE CONTINUED TO FL240 (WE HAD A CLRNC TO FL240). WE RPTED DSNDING THROUGH FL280; ATC ACKNOWLEDGED. AFTER LNDG WE WROTE UP THE AUTOPLT. THE CAUSE OF THIS SEEMS TO BE A MALFUNCTIONING AUTOPLT THAT WE DIDN'T NOTICE RIGHT AWAY BECAUSE OF OTHER DISTRS -- SEATAC CHANGING RWYS; ATC GIVING DELAY VECTORS AND SPD CHANGES. I THINK OUR SITUATIONAL AWARENESS WOULD HAVE BEEN IMPROVED IF ONLY ONE OF US AT A TIME WAS WORKING WITH THE FMS.