B757-200 CREW HAD A DOUBLE MAP SHIFT WHILE FLYING THE DYLIN 2 STAR INTO EWR.

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DOUBLE MAP SHIFT. ON DSCNT TO EWR ON THE DYLIN 2 ARR; FO CROSSED SOMTO INTXN AT 11000 FT. HE WAS DSNDING PRECISELY TO MAKE THE XING ALTS. HE CONTINUED THE DSCNT TO CROSS DYLIN AT 8000 FT. AROUND YARDLEY; WE WERE AT 8500 FT AND DSNDING TO CROSS DYLIN AT 8000 FT WHEN WE EXPERIENCED A SUDDEN MAP SHIFT. THIS SHIFT CAUSED SOMTO TO APPEAR IN FRONT OF US. FO DISCONNECTED THE AUTOPLT AND STARTED A CLB TO 11000 FT. THE MAP SHIFTED AGAIN BECAUSE SOMTO AND DYLIN SUDDENLY APPEARED BEHIND US AND WE MISSED THE DYLIN 8000 FT BY 800 FT BECAUSE OF THE CLB FO STARTED TO 11000 FT. WE WERE INSTRUCTED BY ATC TO TURN TO A 020 DEG HDG AT MERSR AND MAY HAVE MISSED THAT TURN A LITTLE DUE TO THE MAP SHIFT. BY THE TIME WE REVERTED TO RAW DATA; THE MAP APPEARED TO BE NORMAL. ATC SAID NOTHING. THE REMAINDER OF THE FLT WAS NORMAL. FO HAD THE AIRPLANE DSNDING PERFECTLY FOR THE ARR SO WHEN THE SHIFTS OCCURRED; I AM REASONABLY SURE THAT WE MADE THE XINGS AND HDG ASSIGNMENT REGARDLESS OF THE MAP SHIFTS. I HAVE NEVER SEEN THIS TYPE OF SHIFT X2 ALONG THE PATH AND WOULD NOT HAVE BELIEVED IT POSSIBLE IF I HAD NOT SEEN IT MYSELF.