SMT ENG TURBOPROP PLT HAD TRACK AND AIRSPD DEVS WHILE ATTEMPTING LOW MINIMUMS ILS APCH.

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WE WERE IFR INTO THE WASHINGTON; DC AREA; FILED INTO LEESBURG (JYO). WE SHOT THE LOC RWY 17 APCH INTO LEESBURG USING THE AUTOPLT WITHOUT INCIDENT; BUT WERE WELL INTO THE OVCST AND COULD NOT SEE THE RWY; SO WE WENT MISSED. WE THEN ELECTED TO PROCEED TO IAD; OUR FILED ALTERNATE; AND WERE VECTORED ONTO A R DOWNWIND FOR RWY 1R ILS. EVERYTHING WAS NORMAL. THE AUTOPLT WAS FLYING ON ALT HOLD AND HDG HOLD (KFC 300 FEEDING A SANDEL EHSI). THE CTLR VECTORED US ONTO A MODIFIED R BASE; THEN AN INTERCEPT HDG FOR THE LOC; BUT WAS BUSY AND TURNED US LATE SO WE OVERSHOT THE LOC. HE GAVE US A NE HDG TO INTERCEPT FROM THE W; AND I SELECTED 'APPR' MODE ON THE AUTOPLT. WHEN THE AUTOPLT CYCLED FROM 'APR ARM' TO 'APR CPLD;' THE AUTOPLT BANKED SHARPLY TO THE R; DIRECTING A TURN THROUGH THE LOC. AT THAT POINT; I DISCONNECTED THE AUTOPLT; AND TURNED BACK TOWARDS THE LOC. I DID NOT TURN OFF THE FLT DIRECTOR. XA00 IS A BUSY TIME FOR ARRS AT DALLAS; THE WX WAS LOW IFR; AND EVERYONE WAS BUSY; SO THE PRESSURE TO COMPLETE THE APCH WAS HVY. FOR SOME REASON; I FIXATED ON THE FLT DIRECTOR STEERING COMMAND BARS. AS I FLEW THROUGH THE LOC FROM E TO W; THE FLT DIRECTOR COMMANDED A TURN TO THE L; AWAY FROM THE LOC. AS I TURNED RIGHT BACK TO THE LOC; THEY COMMANDED A R TURN. THE COMMAND BARS RACKED BACK AND FORTH VIOLENTLY WITH HDG CHANGES; LOOKING LIKE A MAN TRYING TO CHOP DOWN A TREE WITH A 2-BLADED AXE. AFTER TURNING THROUGH THE LOC A SECOND TIME; DALLAS LCL CAME ON THE RADIO; AND TOLD ME THAT IT APPEARED THAT I WAS L OF THE LOC (PARALLEL APCHS TO RWY 1L WERE IN PROGRESS). I TURNED BACK TO THE R; IGNORING THE FLT DIRECTOR. AT THAT POINT; I THREW IN THE TOWEL; AND ASKED FOR A R TURN OFF TO THE E TO START OVER AND TRY TO FIGURE OUT WHAT WAS GOING ON. THEY VECTORED ME ONTO A R DOWNWIND; WITH A TURN ONTO FINAL 5 MI OR SO OUTSIDE THE APCH GATE; SO I HAD PLENTY OF TIME TO SET UP FOR A MANUAL; SECOND APCH. I SET THE SANDEL TO A NO-MAP MODE; AND HAND FLEW THE APCH. AT THAT TIME; I NOTICED THAT THE COURSE BAR WAS NOT SET ON 010; AND I HAD TO SLEW IT MANUALLY. THE CTLR ALSO ISSUED A 170 KT RESTR TO THE FAF. ON FINAL; I DID SLOW BELOW 170 KTS; BUT CORRECTED BEFORE REACHING THE FAF. THE NEXT MORNING; I CALLED MY INSTRUCTOR AND ASKED HIM FOR HIS EVALUATION AND ADVICE. HE MENTIONED THAT THE PROB MAY HAVE BEEN INTERFERENCE FROM THE RWY 1L ILS; AND THAT THERE WAS PROBABLY NOTHING WRONG WITH THE AIRPLANE. TODAY; I CALLED GARMIN TO DISCUSS MY ADVENTURE WITH THEM. THEY TOLD ME THAT; SINCE THE CDI (COURSE DEV INDICATOR) WAS SET ON VOR/LOC THE WHOLE TIME; THE COURSE BAR ON THE SANDEL PROBABLY DID NOT AUTOMATICALLY SLEW TO THE FINAL APCH COURSE AT DALLAS; BUT REMAINED ON THE COURSE AT LEESBURG. THIS MAKES MORE SENSE; AS THE AUTOPLT/FLT DIRECTOR JUST KEPT TRYING TO TURN AROUND THE SHORTEST CIRCLE TO THE NEARLY RECIPROCAL COURSE AT LEESBURG. AS THE TAIL OF THE ACFT SWUNG THROUGH THAT COURSE; THE COMMAND BARS WOULD RACK BACK TO THE OPPOSITE SIDE; POS RELATIVE TO THE LOC NOTWITHSTANDING. WHAT I DID WRONG: FOLLOWED THE COMMAND BARS TO THE EXCLUSION OF ALL OTHER INDICATIONS AND COMMON SENSE. ALL I HAD TO DO AT ANY POINT IN THE FIASCO WAS TO LEVEL THE WINGS; LOOK AT THE HDG AND LOC CDI; AND CORRECT ACCORDINGLY. WHAT I DID RIGHT: ADMITTING DEFEAT; 'FESSING UP TO DALLAS LCL; AND ASKING FOR THE TURN TO THE E TO SORT THINGS OUT.