A B737-800 FMS DATABASE CORRUPTION CAUSED THE DFW NOBLY 2 LIT TRANSITION CORTS WAYPOINT TO DROP OUT AFTER TKOF. THE CREW VERIFIED THE ROUTE ON PREFLT.

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PRIOR TO DEP ON NOBLY2 LIT; RNAV DEP; RTE AND LEGS WERE CHKED TWICE; ONCE ON PREFLT; AND AGAIN ON TAXI-OUT AFTER CLOSEOUT RECEIVED (THIS CAPT'S PERSONAL STANDARD PROC); ALL LEGS AND SEQUENCING VERIFIED CORRECT AND CHART OUT AND READY. CORRECT SEQUENCE: NAVYE-JGIRL-CORTS-NOBLY; DISPLAYED ON NAV DISPLAY. AFTER TKOF ACFT FLOWN MANUALLY; LNAV ENGAGED. PASSING JGIRL; FMC DROPPED CORTS AS WE TURNED WITH NO DISCONTINUITY OR EXECUTE LIGHT. BY THE TIME WE RELOADED AND REFERRED TO CHARTS FOR CORRECT SEQUENCE WE HAD STRAYED 3 MI N OF RNAV TRACK. ATC ALSO QUERIED DURING THIS TIME AS WE RETYPED CORRECT LEGS BACK INTO BOX. WE CORRECTED BACK TO CORTS-NOBLY LEG; AS QUICKLY AS IT IS POSSIBLE TO TYPE INTO LEGS. I HAVE NOT SEEN THE FMS DO THIS BEFORE AND IT IS IMPOSSIBLE TO CATCH IN TIME AT 250 KTS ONCE THE TURN HAS STARTED; AS CREW ATTN IS OUTSIDE CLRING FOR TFC AND WE WERE GIVEN CLRNC TO 17000 FT WHICH BOTH PLTS WERE CONFIRMING. I TOOK A PHOTO IMMEDIATELY TO SHOW DEP/ARR PAGE AND RTE PAGE WITH CORRECT LOADING STILL ACTIVE; BUT NO CORTS THERE ANYMORE. 'ATC RTE' WAS STILL SHOWN ON LEGS; NOT 'MOD RTE.' WE APOLOGIZED TO ATC. ON ARR INTO ZZZ ON RNAV ARR; WE WERE WATCHING THE ACFT CAREFULLY; WHEN IT DID IT AGAIN; AND DROPPED THE ACTIVE WAYPOINT WITH ABOUT 5 MI TO RUN AND SEQUENCED TO A POINT DOWN-RTE; WITH 'RESYNCING' DISPLAYED IN LEGS. WE INTERVENED AND FLEW TO CORRECT POINT; WROTE UP THE FMC ON ARR. MAINT FOUND LATEST NAV DATABASE INCORRECTLY LOADED OR CORRUPTED; AND RE-LOADED DATA BEFORE NEXT FLT; WITH NO RECURRENCE OF ANOMALY.