ACFT EQUIP PROBLEM WITH FLT GUIDANCE SYS CREATES AN ELECTRICAL ELECTRONIC PANEL PROBLEM FOR FLT CREW DURING IAP ILS APCH.

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THE FO WAS FLYING THE ACFT; AT NIGHT IMC. APPROX 10 SECONDS AFTER CAPTURING THE LOC; THE AUTOTHROTTLES AND AUTOPLT KICKED OFF; AS WELL AS THE FLT GUIDANCE CTL PANEL AND FLT MODE ANNUNCIATOR (FMA) GOING BLANK. I TOOK CTL OF THE ACFT AND STABILIZED THE APCH USING MY FLT GUIDANCE AND FMA; (AS WE WERE IN A 25-30 DEG BANK; DSNDING AT 1000-1500 FPM). I THEN GAVE THE ACFT BACK TO THE FO; WHO REENGAGED THE AUTOTHROTTLES AND AUTOPLT. IN APPROX 2-5 SECONDS; THE DEVICES AGAIN KICKED OFF AND THIS TIME THE FMA SHOWED 'PWR UP TST BX2.' (ACCORDING TO OUR FLT MANUAL THIS SHOULD ONLY HAPPEN AFTER THE ACFT HAS BEEN ON THE GND FOR 2 MINS). I TOOK THE ACFT AGAIN AND FLEW IT UNTIL BECOMING VMC; THEN GAVE THE ACFT BACK TO THE FO; WHO COMPLETED THE APCH AND LNDG VISUALLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. THIS WAS HIS FIRST EXPERIENCE WITH THIS TYPE OF AUTOPLT DISCONNECT/FAILURE WITH LOSS OF THE FO'S LOSS OF SECONDARY FLT INFO. FO DID NOT LOSE PRIMARY FLT INFO SUCH AS HDI; ETC; BUT DID LOSE DIGITAL READOUT INFO ON NAV FREQ; ACTUAL ALT ETC. APPEARS TO RPTR TO BE A FLT GUIDANCE COMPUTER PROBLEM THAT KICKED THE AUTOPLT AND AUTOTHRUST LEVERS OFF.