B737-200 ON VECTOR FOR ROSSLYN LDA RWY 18 APCH. LOC INDICATOR SHOWED N OF COURSE VERY RAPIDLY. CHKED WITH CTLR HE SHOWED ON COURSE. ELECTED TO MAKE A GAR AND WAS CLRED TO 3000 FT. OVERSHOT ALT BY 400 FT. CTLR ADMITTED TO ANOTHER ACFT HE WAS SHOWING THE WRONG INTXN.

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WE WERE BEING VECTORED FOR THE ROSSLYN LDA RWY 18 AT DCA FROM THE S AND GIVEN A HEADING AND ALT TO INTERCEPT. LOC COURSE INDICATOR MOVED QUICKLY TO THE R INDICATING WE WERE NOW N OF COURSE. WE THEN TOOK UP A HEADING TO REINTERCEPT FROM THE N; HOWEVER WE WERE NOT ABLE TO ESTABLISH OURSELVES BACK ON COURSE. I (FO) QUERIED APCH IF THEY SHOWED US N OF COURSE. THEY SAID THEY SHOWED US ON COURSE AT THE SAME TIME WE WERE SHOWING FULL SCALE DEFLECTION AND TWICE HAD AN 'OFF' FLAG APPEAR MOMENTARILY. AT THIS POINT WE DECIDED TO EXECUTE A GAR AND ADVISED ATC. WE WERE GIVEN A HEADING AND TOLD TO MAINTAIN 3000 FT. ON THE GAR WE OVERSHOOT 3000 FT BY 400 FT. WE THEN DSNDED TO 3000 FT. SUGGESTION: WATCH ALT CTL MORE CLOSELY WHEN MAKING A GAR FROM LESS THAN 1000 FT BELOW LEVEL OFF ALT. SUPPLEMENTAL INFO FROM ACN 354195: ANOTHER ACFT THEN QUESTIONED CTLR AS TO INTXN HE WAS OVER. CTLR LATER ADMITTED HE WAS MISTAKEN AND AGREED WITH PLT. SUSPECT CTLR WAS USING WRONG APCH OVERLAY OR ASSUMED HE WAS ASSIGNING A DIFFERENT APCH.