WX NOT AS RPTED AND ERRATIC GS INDICATION RESULTS IN MISSED APCH AT MYR; SC.

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THE FLT WAS ROUTINE UNTIL THE DSCNT INTO MYR. DURING THE DSCNT; I LOST MY SITUATIONAL AWARENESS AND ENDED UP HIGH ON THE APCH. I HAD TO EXECUTE A MISSED APCH PROC. DURING THE DSCNT; THE AUTOPLT WAS ENGAGED UNTIL AFTER GS CAPTURE. AFTER GS CAPTURE; THE GS INDICATOR FLUCTUATED FULL DEFLECTION UP AND DOWN RAPIDLY. THE AUTOPLT WAS TRYING TO FOLLOW THE GS INDICATIONS WHICH RESULTED IN LARGE PITCH ATTITUDE CHANGES FROM A FEW DEGS NOSE DOWN TO 15 DEGS NOSE UP. AT THIS POINT I DISCONNECTED THE AUTOPLT AND OVERCOMPENSATED FOR THE NOSE UP ATTITUDE WITH AN INCREASE IN THRUST AND ATTITUDE ADJUSTMENT THAT RESULTED IN AN INDICATED AIRSPACE OF 220 KIAS. AS WE WERE PASSING THROUGH 1600 FT; WE SAW THE RWY THROUGH THE BROKEN CLOUD LAYER; REALIZED WE WERE TOO HIGH AND GOING TOO FAST; AND A MISSED APCH WAS CARRIED OUT. CONTRIBUTING FACTORS: ATIS WX 10000 FT BROKEN; VISIBILITY 4 MI; LIGHT RAIN; CALM WINDS; RWY 17. ACTUAL WX 800 FT BROKEN; VISIBILITY 6 MI IN HAZE; CALM WINDS. THE CLOUDS OBSCURED OUR VIEW OF THE ARPT AND I DID NOT REALLY MENTALLY PREPARE MYSELF FOR AN IFR APCH; EVEN THOUGH WE CARRIED OUT A BRIEFING AND PROPERLY SET UP THE NAVAIDS. ALSO; AFTER BEING CLRED FOR THE APCH; WE MUST HAVE CAPTURED A FALSE GS WHICH LED TO THE ERRATIC GS INDICATIONS AND LARGE PITCH ATTITUDE CHANGES. WHAT REALLY CAUSED THE PROB: MY INATTN TO DETAIL ON THIS FLT LED TO LOSS OF SITUATIONAL AWARENESS WHICH LED TO THE HIGH AND FAST APCH. CORRECTIVE ACTION: WHEN ON DUTY; MAINTAIN AN APPROPRIATE LEVEL OF ALERTNESS AT ALL TIMES TO AVOID A FUTURE RECURRENCE.