FO OF A B737-400 ALLOWED THE ACFT TO ASCEND ABOVE ASSIGNED ALT WHEN SLOWING THE ACFT FOR DSCNT DUE MISPROGRAMMING THE FMC AUTO HOLD BEFORE IT WAS TIME FOR THEIR DSCNT.

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ZSE WAS SLOWING US DOWN AND GIVING US VECTORS FOR SPACING INTO SEATTLE. APPROX 40 MI FROM TOP OF DSCNT; WE WERE CLRED; PLT'S DISCRETION; TO FL240; WHICH WE SET IN THE ALT ALERTER. THE FO WAS PF AND RESPONDED TO ATC AS I WAS OFF FREQ TO GET ATIS. A FLT ATTENDANT CAME UP AT THAT TIME TO TAKE AWAY OUR MEAL TRAYS AND REMIND US THAT A MEDICAL OXYGEN BOTTLE WAS NEEDED IN SEATTLE ON LNDG. WHEN I TURNED FORWARD; I NOTICED THAT WE WERE TURNING BACK TO BTG PER ATC INSTRUCTIONS; BUT THE AUTOPLT WAS IN CTL WHEEL STEERING PITCH MODE. A QUICK GLANCE AT THE ALT SHOWED THAT WE WERE CLBING AND PASSING FL316. (ASSIGNED ALT WAS FL310.) I GRABBED THE CTLS AND GENTLY PUSHED THE ELEVATOR OVER TO GET US BACK TO FL310. OUR ALTIMETER SHOWED WE GOT TO FL318 BEFORE WE STARTED BACK DOWN. NO ALT ALERTER WENT OFF BECAUSE WE HAD FL240 IN THE MCP. I AM NOT SURE HOW THE ACFT GOT INTO CTL WHEEL STEERING PITCH MODE; BUT THE LACK OF ANY BELLS; MY ATTN DIVERTED TO THE FLT ATTENDANT; BUSY ATC INSTRUCTIONS; AND THE FO NOT NOTICING WHAT MODE THE AIRPLANE HAD DOWNGRADED TO; CAUSED US TO BUST OUR ALT.