A VERY LOW TIME PLT; FLYING A VERY COMPLEX ACFT; RPTED A SERIES OF ERRORS DURING A SHORT FLT FROM PAE TO TIW.

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I WAS DELIVERING MY PLANE; A RAYTHEON/BEECH A36; FROM PAE (PAINE FIELD; EVERETT; WA) TO THE SHOP AT TIW (TACOMA NARROWS; GIG HARBOR; WA; 40 NM S) FOR ITS ANNUAL INSPECTION. IT WAS A DARK AND STORMY NIGHT; RAINING; WITH A STRONG (20-40 KTS) HEADWIND; LIGHT TO MODERATE TURB; 5-7 MI VISIBILITY; 700-1500 FT SCATTERED TO BROKEN; 3000-4000 FT OVCST. ALTHOUGH SHORT; THIS IFR FLT GOES INTO; THEN BELOW; THE SEATTLE CLASS B AIRSPACE AND IS QUITE BUSY WITH FREQ AND ALT ASSIGNMENTS. UNDER THESE CONDITIONS; ESPECIALLY THE TURB; THE WORKLOAD WAS HIGH FOR A SINGLE PLT. AT ONE POINT; I FELT MYSELF GETTING BEHIND THE AIRPLANE SO I REDUCED PWR AND SLOWED THE PLANE DOWN FROM 160 KTS TO 120 KTS TO GIVE MYSELF MORE TIME. THIS WORKED; IT CAUGHT ME UP WITH THE AIRPLANE; BUT I FEEL IT CONTRIBUTED TO WHAT CAME NEXT. SEA APCH VECTORED ME TO INTERCEPT THE TIW LOC FOR RWY 17; THEN CLRED ME TO DSND TO AND MAINTAIN 2000 FT MSL UNTIL GS INTERCEPT AT THE FAF; 11 MI AHEAD. BOTH NAV #1 AND #2 WERE DIALED IN TO THE ILS FREQ; I WAS HAND FLYING ON THE FLT DIRECTOR. AFTER A FEW MINS; THE GS INDICATOR OF THE #1 NAV APPEARED AT THE TOP OF THE HSI; THEN SMOOTHLY; GRADUALLY DSNDED TO 1/2 DOT ABOVE THE CTR; WHERE I LOWERED THE GEAR; BEGAN MY DSCNT; AND CONTACTED TIW TWR. THE TWR SAID THAT HIS RADAR SHOWED NO TARGET AT THE FAF; ONLY ONE STILL 7 MI OUT FROM THE FAF. AT THAT POINT; I SAW THAT THE #2 GS STILL SHOWED THE GS FULL SCALE ABOVE ME. I HAD NOT REACHED THE FAF AFTER ALL; AND HAD BEGUN MY DSCNT (AND SWITCHED FREQS) TOO SOON. I WAS AT ABOUT 1500 FT MSL; 500 FT BELOW MY LAST ASSIGNED ALT AND ONLY ABOUT 100 FT AGL. THE HSI GS INDICATOR HAD FAILED; RESPONDING TO ANY GS SIGNAL BY ALWAYS INDICATING 1/2 DOT BELOW THE GS. THE PRIMARY CAUSE OF THIS ALTDEV WAS THE HSI FAILURE; BUT I HAD ENOUGH INFO AVAILABLE TO KNOW THAT I WAS NOT AT THE FAF; HAD I LOOKED. DISTANCE TO THE ARPT WAS DISPLAYED ON THE GPS AND THE APCH PLATE GAVE THE DISTANCE FROM THE FAF TO THE THRESHOLD; NOT EXACTLY THE SAME MEASURE; BUT CLOSE ENOUGH TO SHOW THAT I WAS ALMOST 3 TIMES FARTHER AWAY THAN I THOUGHT. I COULD HAVE USED THE XRADIAL TO IDENT THE FAF; BUT I HAD TUNED BOTH NAVS TO THE LOC FREQ. ANOTHER FACTOR WAS THE UNUSUALLY LOW GND SPD; DUE TO THE REDUCED PWR AND STRONG HEADWIND; SO THE TIME ELAPSED TO WHAT I THOUGHT WAS THE FAF DIDN'T SEEM TOO SHORT. THE HSI HAD EXHIBITED THIS GS FAILURE MODE TWICE BEFORE INFLT; BUT ALWAYS PERFORMED FLAWLESSLY IN THE SHOP; SO THE PROB HAD NOT BEEN FOUND OR FIXED. IN ADDITION; BOTH OF THE PREVIOUS FAILURES HAD BEEN SUDDEN AND OBVIOUS; SNAPPING QUICKLY TO THE 1/2 DOT LOW INDICATION; LEADING ME TO THE BELIEF THAT I COULD IDENT WHEN IT HAD FAILED BY WATCHING IT; A BELIEF THAT WAS OBVIOUSLY WRONG. UPON RETURNING TO MY ASSIGNED ALT OF 2000 FT; I TURNED ON THE AUTOPLT AND SET IT TO HOLD ALT WHILE I RECONFIGURED THE RADIOS. WHEN I LOOKED UP AGAIN; THE AUTOPLT HAD DEVIATED ABOVE MY ASSIGNED ALT; CLBING TO 2300 FT. THE APCH MODE HAD SUPERSEDED THE ALT HOLD AND SINCE THE HSI WAS INDICATING 1/2 DOT BELOW THE GS; THE AUTOPLT BEGAN CLBING IN AN ATTEMPT TO CTR IT. AT THAT POINT; I TURNED OFF THE AUTOPLT/FLT DIRECTOR ALTOGETHER; RETURNED TO MY ASSIGNED ALT; AND HAND FLEW THE REMAINDER OF THE APCH ON THE #2 NAV/GS. SINCE I WAS DELIVERING THE PLANE FOR ITS ANNUAL INSPECTION ANYWAY; I ADDED THIS HSI PROB TO THE LIST OF THINGS TO BE CHKED. UNTIL THE PROB WITH THE HSI IS FOUND AND FIXED; ILS APCHS WILL BE FLOWN ONLY WITH THE #2 NAV/GS; WITHOUT THE AUTOPLT; FLT DIRECTOR OR HSI. FINALLY; I LEARNED THAT ANY COMPONENT THAT IS KNOWN TO FAIL INTERMITTENTLY CAN NOT BE RELIED UPON EVEN WHEN IT APPEARS TO BE BEHAVING. ANYTHING THAT IS WORKING; SOMETIMES MUST BE CONSIDERED BROKEN; ALWAYS.