AN E120 CREW ON APCH TO IAH OVERSHOT ASSIGNED ALT; DEVIATED FROM ASSIGNED HDG.

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FO WAS PF FROM EPD TO IAH. WE WERE STABLE AT 3000 FT ON A HDG OF 100 DEGS VECTORED BY HOUSTON APCH FOR THE ILS RWY 27. ATC ADVISED US TO SLOW FROM 230 KTS TO 180 KTS AND TURN L TO A HDG OF 280 DEGS TO INTERCEPT THE LOC INBOUND. ASSIGNED ALT WAS 3000 FT. FO COMPLIED BY REDUCING PWR AND INITIATING A L TURN. (AUTOPLT WAS OFF FOR THIS FLT BY FO'S CHOICE.) I DOUBLECHKED LNDG SPDS; AND ATIS INFO ON DATA CARDS. WHILE LOOKING AT THIS INFO; I HEARD THE ALT ALERT SOUND A WARNING. I LOOKED TO SEE US IN A 34 DEG L BANK WITH NOSE DOWN APPROX 8-9 DEGS. ALT WAS RAPIDLY DECREASING PAST 2600 FT. I LOUDLY STATED 'CHK ALT.' FO RESPONDED WITH 'OK...CORRECTING;' BUT MADE NO MOVE. IT APPEARED THE FO WAS NOT MOVING. I ANNOUNCED AGAIN 'ALT!' AND MOVED THE YOKE TOWARDS WING LEVEL AND NOSE UP. FLT DIRECTOR SHOWED FO FLYING AWAY FROM DESIRED PATH. AT LEVELOFF OUR ALT WAS THEN 2000 FT. I CONTACTED ATC AND ADVISED THAT WE COULD CLB IF ATC SO DESIRED. ATC ADVISED THAT ALT WAS FINE AND TO SLOW 180 KTS. WE CONTINUED APCH AND LANDED WITHOUT FURTHER INCIDENT. FO APOLOGIZED AND STATED 'I DON'T KNOW WHAT HAPPENED BACK THERE.' I BELIEVE THE FO BECAME DISORIENTED WHILE IN THE TURN AND FAILED TO UNDERSTAND OR RESPOND TO THE SIT. I SHOULD HAVE TAKEN THE YOKE IMMEDIATELY AT THE FIRST CHALLENGE. TOTAL TIME START TO FINISH APPROX 20-25 SECONDS.