A319 CREW HAD TRACK DEV DEPARTING SAN RWY 27.

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WE PROGRAMMED OUR FLT MGMNT COMPUTER FOR OUR CLRED RTE INCLUDING THE SID (BORDER FOUR DEP). BOTH FO AND MYSELF DOUBLE-CHKED THE RTE AND BOTH OF US CONFIRMED IT AS CORRECT. DEP RWY WAS RWY 27. AT APPROX 400 FT; THE FO CALLED FOR AUTOPLT #2 ON. I SELECTED AUTOPLT #2 ON. ON MY FMA; 'NAV ENGAGED AND AUTOPLT #2 ON.' HOWEVER; AT THIS POINT THE ACFT BEGAN A TURN TO THE L. THIS DEP REQUIRES A HEADING OF 270 DEGS UNTIL REACHING THE PGY 19 DME. I NOTICED SOMETHING WAS WRONG AND I TOLD FO TO DISCONNECT THE AUTOPLT AND TURN THE ACFT TO THE R TO HEADING OF 275 DEGS. I SELECTED 275 DEG HDG ON FLT CTL UNIT AND WE TURNED TO 275 DEGS. AT THIS POINT; WE WERE SLIGHTLY L OF A NORMAL DEP TRACK. ON OUR NAV DISPLAYS; WE HAD ALL OF OUR WAYPOINTS DISPLAYED; BUT NO COURSE LINE TO CONNECT THE WAYPOINTS. WE FLEW THE DEP MANUALLY UNTIL WE WERE ABLE TO ESTABLISH A COURSE LINE ON OUR NAV DISPLAYS. ONCE CLRED TO THE NEXT FIX; WE WERE ABLE TO ESTABLISH NORMAL NAV CAPABILITIES. LOOKING BACK; I REMEMBER OUR COURSE LINE BEING THERE JUST PRIOR TO TKOF. SOMETIME BTWN TKOF AND 400 FT THE FAULT OCCURRED. WE CHKED OUR SECONDARY FLT PLAN AND IT CONTAINED THE EXACT DEP DATA SO NO ERROR WAS MADE IN ENTRY. SAN DIEGO IS VERY NOISE SENSITIVE; SO STRICT ADHERENCE TO THE DEP IS A MUST. WE WERE SLIGHTLY S OF THE NORMAL PATH. ONCE THE ACFT BEGINS A TURN IT TAKES A BIT TO TURN IT THE OPPOSITE WAY. CURRENTLY SEARCHING FOR AN ANSWER FROM TECHNICIANS. OUR SOMEWHAT ERRATIC TRACK WAS NOT MENTIONED BY ATC (SOCAL DEP OR SAN DIEGO TWR).