A B737-500 CREW; IN DSCNT TO BOS; OVERSHOT THEIR ASSIGNED ALT.

Narrative

SECOND LEG RETURN TO BOS FROM EWR. CAPT FLYING WITH AUTOPLT ON; VNAV AND LNAV ENGAGED; DIRECT TO PVD VOR. ATC INSTRUCTED AIRSPD INCREASE TO 320 KIAS OR GREATER TO CROSS PVD AT 11000 FT. WITH 100 KT TAILWIND; CAPT LEFT FL210 EARLY ON VNAV DSCNT. PASSING THROUGH FL180; IN-RANGE CHKLIST ACCOMPLISHED; ALTIMETER 29.46 WITH CURRENT BOS ATIS. ABOUT 10 NM PRIOR; ATC INSTRUCTED US TO CROSS PVD AT 12000 FT AND GAVE HOLDING INSTRUCTIONS OVER PVD. CAPT RESET ALT TO 12000 FT IN MCP. THE CAPT PROGRAMMED THE HOLDING PATTERN IN THE FMC WITH MY HELP TO MAKE THE CORRECT ENTRIES. TOOK A BIT LONGER THAN NORMAL AND WE WERE RAPIDLY APCHING PVD VOR ATA HIGH AIRSPD AND RATE OF DSCNT. AT APPROX 330 KIAS; THE CLACKER SOUNDED AND I LOOKED TO SEE THE AIRSPD HIGH AND WITHIN 100-200 FT OF 12000 FT ASSIGNED ALT. I CALLED 'ALT; ALT; WE NEED TO LEVEL.' CAPT WAS BUSY WITH FMC ENTRIES TO HOLDING PATTERN. I TOOK THE YOKE AND STOPPED THE DSCNT 400-500 FT BELOW 12000 FT AND IMMEDIATELY RETURNING TO CORRECT ALT I THEN NOTICED BOTH CAPT'S ALTIMETERS SET TO 30.46 SHOWING 1000 FT DIFFERENCE; WHICH HE THEN CORRECTED. ATC DID NOT CALL US ABOUT DEV. MONITORING PLT DID NOT DOUBLECHK ALTIMETER SETTING FOR THE PF ON THE IN-RANGE CHKLIST. TOO MUCH ATTN PLACED ON FMC ENTRIES FOR A HOLDING PATTERN DURING A CRITICAL PHASE.