A FLC WAS DISTRACTED BY RADIO TUNING; A NEW IAP CLRNC; AND AN EQUIP MALFUNCTION. THE CLRNC ALT WAS OVERSHOT BY 400 FT BEFORE THE CREW RECOGNIZED THE DEV.

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PHL APCH CTL CLRED US TO DSND AND MAINTAIN 8000 FT; AND DEPART 'BUCKS' INTXN ON A HDG OF 100 DEG. EXPECT ILS RWY 27L; PHL. THE ALT ALERT SELECTOR WAS CORRECTLY SET AT 8000 FT; AND BOTH PLT'S ALTIMETERS WERE SET AT THE CORRECT ALTIMETER SETTING. THE ALT ALERT WARNING SYS WAS FUNCTIONING CORRECTLY AND YET I CONTINUED DSCNT THROUGH 8000 FT; GOT THE ALT ALERT WARNING AT 300 FT LOW; BUT DID NOT ARREST THE DSCNT UNTIL THE FO CALLED 'BELOW 8000 FT.' THIS OCCURRED AT APPROX 7600 FT; INDICATED ALT. WITHIN 10 SECONDS; I HAD RETURNED THE ACFT TO LEVEL FLT; AT 8000 FT; AND RECEIVED NO COMMENT FROM PHL APCH CTL REGARDING AN ALTDEV. IN A POSTFLT DEBRIEF WITH THE CREW WE DETERMINED THAT THE FOLLOWING FACTORS CONTRIBUTED TO OUR MISSING SUCH A CRUCIAL; AND ROUTINE; FLT SIT CHK: 1) WE WERE ALL TIRED. WE'D CHKED IN FOR DUTY AT XC00 LCL PACIFIC TIME FOR THE FIRST LEG OF A 5 DAY PAIRING. AT THE TIME OF THE INCIDENT WE HAD BEEN AIRBORNE FOR 4 HRS AND WERE NOW; FLYING INTO THE MORNING SUN. 2) THE FO WAS NEW TO THE ACFT; AND AT THE TIME WAS PREOCCUPIED WITH TUNING AND IDENTING ILS FREQS. HE WAS; HOWEVER; THE FIRST OF US; TO CATCH THE DEV. 3) THE FE WAS BUSY HANDLING AN ABNORMAL SIT: AN AIRCONDITIONING 'PACK TRIP;' WHICH HAD OCCURRED. SIMULTANEOUSLY; WITH THE DEV. 4) FOR MYSELF: I WAS SIDETRACKED BY THE ILS RWY CHANGE - I EXPECTED; AND BRIEFED FOR RWY 27R. THE CHANGE TO RWY 27L HAD ME SCRAMBLING FOR THE APPROPRIATE CHART.