B757-200 CREW HAD MULTIPLE ALT OVERSHOOTS WHEN FOLLOWING THE FLT GUIDANCE SYS.

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A VISUAL TKOF AT VERY LOW GROSS WT; THIS B757-200 WANTS TO OVERSHOOT ALT WITH JUST THE FLT DIRECTOR COUPLED. I THINK THE AIR DATA COMPUTER JUST CAN'T HANDLE THE FOLLOWING CONDITIONS: LOW GROSS WT; LOW INITIAL (3000 FT AND 4000 FT) ALT AND HIGH RATE OF CLB/CLOSURE RATES. UPON DEP AT SWF THE ACFT MODE ANNUNCIATOR 'CAPTURED' FOR 3000 FT; BUT UPON SELECTING 4000 FT AND FLT LEVEL CHANGE; IT OVERSHOT BY 299 FT. THIS WAS FLT DIRECTOR HAND FLOWN. THE NEXT MORNING IN A 26 HR SUPPLEMENTAL DUTY DAY PART 121; IF YOU CAN BELIEVE THAT; UPON DEP FROM ABE BACK TO SWF WITH THE AUTOPLT COUPLED (TO PREVENT IT AGAIN) THE ACFT OVERSHOT 5000 FT BY 299 FT. IT CAPTURED EITHER 3000 FT OR 4000 FT INITIALLY; BUT ATC GAVE US 5000 FT AND THE AUTOPLT/FLT DIRECTOR OVERSHOT AGAIN. THE FIRST EVENT WAS IN THE VICINITY OF HUO VOR; THE 2ND NEAR FJC. LATER IN THE WEEK; THE ACFT CAPTURED 3000 FT NICELY; BUT UPON SELECTING 5000 FT AND VNAV; IT CAPTURED 3400 FT. VERY SPOOKY! DO B757'S HAVE AIR DATA COMPUTER UPDATES THAT REFINE THEIR CALCULATION ABILITIES?