FMS FAILED TO MAKE PROGRAMMED TURN AT HARRP INTXN.

Narrative

DUE TO HVY WX ON NORMAL ARR RTE (NANCI ARR) WE WERE RE- RTED TO THE EXTOL ARR TO AVOID THE WX. DURING DSCNT; APCHING THE HARRP INTXN; I EXPERIENCED FMC FAILURE. IN A FEW SECONDS; THE FMC CAME BACK ON AND SHORTLY THEREAFTER; THE FLT DIRECTOR (ON MY SIDE ALSO) FAILED. -- I WAS TRYING TO LOCATE THE PROPER CIRCUIT BREAKERS TO RESET THE FLT DIRECTOR WHEN IT SUDDENLY CAME BACK ON. THE ACFT HAD FAILED TO MAKE THE TURN AT HARRP TO LGA AND BOTH THE FO AND I FAILED TO NOTICE DUE TO THE DISTR. APCH CTL POINTED THIS OUT TO US AND THEN GAVE US A VECTOR. THESE TYPE OF ELECTRICAL GLITCHES SEEM QUITE COMMON ON THE MLG; ALTHOUGH I AM NEW ON THE TYPE I'VE ALREADY SEEN SEVERAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR NOW AS 400 HRS ON ACFT. HE SAID THE 'NERVOUS SYS ON THE ACFT IS LOUSY.' A COMMON PROBLEM: THE FMC IS PROGRAMMED TO CROSS A POINT AT AND INTERCEPT AN ALT. IT MAKES THE XING RESTRICTION BUT FAILS TO CAPTURE THE ALT. OFTEN; WHEN A RTE IS RE-ENTERED OR AMENDED; THE FMC SEEMS TO BE OVERLOADED AND IS LOST. HE ADDED THAT THE NAV CTL SYS IS WRITTEN UP ABOUT ONCE PER LEG ON AVERAGE.