DISTRACTION OF UNUSUAL PERFORMANCE OF THE AUTOMATION OF A B717 DURING AN ILS APCH RESULTS IN A SMALL DIVERSION FROM ALT.

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FROM LGA TO KATL WX FORECAST GOOD. A FOG LAYER DEVELOPED AT KATL REQUIRING US TO HOLD. WE WERE WITHIN 5 MIN OF HAVING TO GO TO OUR ALT. THE WX CAME UP AND WE TOOK VECTORS FOR THE RWY 9 R ILS AT ATL. PART OF THE VECTOR WAS TO DSND TO 2700 FT MSL; THIS COINCIDED WITH LOC CAPTURE; WE ALSO WERE TOLD TO SLOW TO 160 KTS. DURING THIS TIME WE HAD AN INTERMITTENT WARNING OF THE ILS UNTUNABLE. AS WE PREPARED TO TAKE ACTION THE ILS TUNED WITHOUT FURTHER INCIDENT (SUPPOSED TO TUNE WITHIN 25 NM). THE PROBLEM WAS THAT FOR SOME REASON DURING THE LOC CAPTURE; SLOW DOWN AND LEVEL OFF; THE AUTOPLT AND AUTOTHROTTLES WERE SLOW TO CAPTURE THE ALT. THIS IS VERY UNUSUAL. AT 200 FT DEV WE HAD AN ALT ALERT FROM THE ACFT. THE CAPT DISCONNECTED THE AUTOPLT AND BEGAN TO CORRECT. AT THIS TIME ATC INFORMED US THAT WE LOOKED LOW. WE RESPONDED AND IMMEDIATELY CORRECTED. I BELIEVE WE GOT 300 FT LOW DURING THE FINAL VECTOR; THE ONLY CONFLICT BEING TERRAIN CLRNC. WE WERE VMC ABOVE A 200 FT THICK FOG LAYER. A VERY AUTOMATED ACFT IS LIKE HAVING A 3RD PLT AND BY DESIGN ALLOWS FOR MORE SITUATIONAL AWARENESS AND EYES OUTSIDE THE COCKPIT. WHEN THE COMPUTER DOES A FLAWLESS JOB 99 TIMES OUT OF 100; YOU ARE CONVINCED IT WILL DO IT EVERY TIME. HOWEVER ONCE AGAIN PROVEN; AT THE WORST POSSIBLE MOMENT; MURPHY'S LAW; OR THE ENGINEER/PROGRAMMER WILL HELP MAKE YOUR DAY! ALSO A FACTOR; THIS WAS A STAND UP WITH 5 HOURS SLEEP.