FLC OF AN MLG FOUND THE GS TO BE ERRATIC DURING AN ILS AUTOPLT COUPLED APCH. WHEN BREAKING OUT OF THE OVCST; THE FLC CONTINUED ON WITH A VISUAL APCH WHILE NOTING A WDB ACFT TAXIING ACROSS THE APCH END OF THE RWY AND BEYOND THE ILS HOLD SHORT POINT LINE.

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LOCATION; ILS APCH TO RWY 17C. ON AN ILS APCH TO RWY 17C WITH THE WX APPROX 1500 FT OVCST; 900 FT AGL; WHILE TRACKING THE LOC AND GS WITH FLT DIRECTOR AND AUTOPLT ENGAGED. THE GS APPEARED TO DIP AND THE ACFT FOLLOWED IT AS WE XCHKED WIND VERSUS ALT VERSUS SINK RATE AND THE DISTANCE TO THE RWY. WE CAME TO THE CONCLUSION THAT SOMETHING WAS SERIOUSLY WRONG; SO THE PNF; FO; PUNCHED OFF THE AUTOPLT AND LEVELED OFF JUST AS WE EXITED THE CLOUD ALERT. WE RPTED THAT THE GS WAS ERRATIC AND THAT THE IDENT SIT OCCURRED YESTERDAY EXACTLY AS THIS ONE DID. FLEW A VISUAL TO A UNEVENTFUL LNDG. WE OBSERVED AN AIR CARGO DC10 ON THE E SIDE OF RWY 17C THRESHOLD; AND AS HE CROSSED IN FRONT OF US; THE GS FLUCTUATED ABOUT 2 DOTS UP AND 2 DOTS DOWN AS WE WERE ON VERY SHORT FINAL THOUGH VFR. AFTER WE RPTED THIS; THE PRECEDING AND FOLLOWING JETS CONFIRMED THIS ALSO. THIS EXACT SCENARIO OCCURRED YESTERDAY MORNING TO US BUT THE WX WAS GOOD; VFR. I STRONGLY RECOMMEND THAT THE PRACTICE OF ALLOWING A LARGE ACFT TO SIT IN THIS LOCATION BE STOPPED AND IF DONE IN VFR; THAT ALL PLTS BE ADVISED OF THIS. SUPPLEMENTAL INFO FROM ACN 339836: BROKE OUT OF 900 FT AGL CEILING AND NOTICED THAT WE WERE LOW FOR THE APCH BUT GS SHOWED RIGHT ON. TWR ADVISED OF THE ALT ALERT AT SAME TIME. WE LEVELED OFF AND FLEW REMAINDER OF APCH IN VISUAL CONDITIONS. LNDG UNEVENTFUL.