CAPT OF MLG ACR ACFT ALLOWED THE ACFT TO UNDERSHOOT DURING DSCNT CAUSING AN ALT XING RESTRICTION NOT TO BE MET. THERE WAS NO KNOWN CONFLICT WITH OTHER TFC.

Narrative

PROBLEM: MISSED XING RESTRICTION ALT. SUCCESSIVE DSCNTS AND XING RESTRICTIONS WITH EACH CHANGING CTLR. HAD BEEN CLRED TO FL190; TO CROSS 30 NM S OF SIE VOR AT 15000 FT. RESTRICTION HAD BEEN PROGRAMMED INTO FMC COMPUTER; BUT I MUST NOT HAVE PROPERLY ENGAGED THE COMPUTER. WHEN CHANGED TO FREQ 127.70; THE CTLR QUERIED OUR CLRNC TO 15000 FT. I CONFIRMED IT -- BELIEVING I STILL HAD APPROX 30 MI TO GO -- BUT IN FACT; I WAS ABOUT 3 MI FROM THE XING. CTLR SAID OK -- JUST DSND TO 15000 FT; WHICH I DID. CONTRIBUTING FACTORS: REPEATED DIFFICULTY HAD BEEN EXPERIENCED WITH SETTING LOWER ALTS INTO THE FMC TO SATISFY EACH NEW RESTRICTION. SOMETIMES WITHOUT SUCCESS. OFTEN WHEN MAKING DSCNTS WITH RAPIDLY CHANGING PARAMETERS; THE AUTOMATED COCKPIT BECOMES UNWIELDY -- AND BECOMES A DISTR TO FLYING THE ACFT. HUMAN PERFORMANCE CONSIDERATIONS: FLT WAS LAST LEG OF A 4-DAY TRIP -- ACCUMULATING 25+ HRS IN THE LAST 80 HRS. A LAYOVER OF ABOUT 3 HRS WAS EXPERIENCED BTWN PREVIOUS AND CURRENT FLT. A CERTAIN AMOUNT OF FATIGUE WAS FELT -- PLUS ANGER AT KNOWING CREW WAS SUBJECT TO POSSIBLE DRUG TEST ON ARR -- COMBING WITH FRUSTRATION AND INABILITY TO MANAGE THE FMC -- ADDED TO THE DISTR OF ADHERING TO THE CLRNC.