AUTOFLT DISTRACTIONS RESULT IN A DEV BELOW PUBLISHED APCH ALT ON APCH TO SAN; CA.

Narrative

ON DSCNT INTO SAN (RWY 27) WE WERE HELD TO 6000 FT AND 180 KTS ASSIGNED UNTIL JUST OUTSIDE VYDDA. WE WERE THEN CLRED THE RNAV RWY 27 APCH. I SELECTED OPEN DSCNT (IDLE) TO GET DOWN TO 4000 FT; THEN 3600 FT (SWATT). TWICE DURING THIS DSCNT; THE DSCNT MODE SWITCHED TO A VERT SPD DSCNT (1000 FPM). AFTER THE SECOND MODE SWITCH OR CHANGE; AND BEING QUITE HIGH ON GLIDE PATH; I TURNED THE AUTOPLT AND BOTH FLT DIRECTORS OFF IN ORDER TO CAPTURE THE GLIDE PATH. I HAD GND CONTACT AT ALL TIMES. THE ARPT WAS IMC; BUT ON FINAL I HAD VMC CONDITIONS. ONCE THE SPD WAS BACK AND I WAS FULLY CONFIGURED I DSNDED THROUGH THE GLIDE PATH. THE GLIDE PATH WENT FROM FULL SCALE HIGH TO FULL SCALE LOW ALMOST IMMEDIATELY. THE FO CALLED THE GLIDE PATH OUT AND I LEVELED OFF AT APPROX 1600 FT. THIS WAS AT REEBO. TWR CALLED WITH A LOW ALT ALERT. I RE-ENGAGED THE FLT DIRECTORS AND CAPTURED THE GLIDE PATH AND CONTINUED THE APCH. CONTRIBUTING FACTORS WERE BEING HELD HIGH ON ALT; AUTOMATION PREVENTING FROM OPEN DSCNT (IDLE) TO A VERT SPD MODE TWICE; AND MY FAILURE TO REALIZE THE DIFFERENCE BTWN A GS AND A GLIDE PATH WHEN I WAS TRYING TO CAPTURE THE GLIDE PATH FROM ABOVE.