A300 AIRBUS FLT CREW HAS TRACK HDG DEV DURING BORDER 5 DEP FROM SAN.

Narrative

DEPARTED RWY 9 IN NAV MODE ON THE BORDER 5 SID. BOTH FMS AND COMMERCIAL CHART SID DEPICTION SHOWED 'AFTER TKOF FLY HDG 090 DEGS TO INTERCEPT THE MZB 096 DEG RADIAL TO BROWS INTXN.' TKOF WAS NORMAL AND FMS WAS DIRECTING A 090 DEG HDG AS PER THE SID. AT ABOUT 2700 FT MSL; THE ACFT DEVIATED FROM THE PROGRAMMED FLT PATH AND STARTED A L-HAND TURN BACK TO 090 DEGS. AT THE SAME TIME SOCAL DEP ASKED IF WE WERE IN A L-HAND TURN. I RESPONDED THAT WE WERE CORRECTING BACK TO THE R. SOCAL ASKED WHAT THE PROB WAS BECAUSE THE PREVIOUS AIRBUS (THAT DEPARTED 15-20 MINS BEFORE US) HAD DONE THE SAME THING. I REPLIED THAT I THOUGHT THAT IT WAS AN FMS OR FLT DIRECTOR PROB. SOCAL THEN GAVE US DIRECT BROWS INTXN. PRIOR TO DEP; THE FLT PLAN PAGE SHOWED THE 090 DEG HDG TIL INTERCEPTING THE RADIAL. WHEN THE ACFT STARTED DEVIATING FROM THE 090 DEG HDG; THE FO'S NAV DISPLAY SHOWED A LINE (EITHER HDG OR RADIAL; WE WEREN'T SURE WHICH) TO THE L OF OUR POS. THE CAPT'S NAV DISPLAY ONLY SHOWED BROWS INTXN. FLT STANDARDS IS LOOKING INTO A PROBABLE CAUSE FOR THE FMS DEV. CREW COULD BE MORE VIGILANT AND QUICKER TO RESPOND WHEN ACFT DEVIATES FROM THE PROGRAMMED FLT PATH.