AT 2500 FT A 319-100 BEGINS UNCOMMANDED CLB DURING APCH TO AUS.

Narrative

I WAS THE PNF. WE WERE LEVEL AT 2500 FT MSL; TRACKING THE LOC FOR THE ILS RWY 35L AUS. APPROX 8 MI FROM THE ARPT; THE AUTOPLT AND AUTOTHRUST WERE ENGAGED. AFTER BEING CLRED FOR THE APCH; THE CAPT ARMED THE APCH AND I SET THE MISSED APCH ALT OF 3000 FT. ABOUT 10 SECONDS LATER; THE ACFT BEGAN AN UNCOMMANDED CLB; THERE WERE NO ABNORMAL INDICATIONS ON THE FMA OR ECAM. I BELIEVE THE ACFT WAS TRYING TO CAPTURE THE GS FROM BELOW. AT ABOUT 3000 FT; THE CAPT DISCONNECTED THE AUTOPLT AND AUTOTHRUST AND BEGAN A DSCNT BACK DOWN TO 2500 FT; CONTINUING THE APCH WITHOUT ANY FURTHER ABNORMALITIES. THERE WERE NO QUESTIONS FROM ATC. SUPPLEMENTAL INFO FROM ACN 458843: CAPT FLYING; IMC IN CLOUDS AND RAIN; LEVEL AT ASSIGNED ALT OF 2500 FT; ON AUTOPLT COUPLED APCH ILS RWY 35L TO AUSTIN. BOTH AUTOPLTS ON; POS 3-5 NM OUTSIDE CREED FAF. FO SAID 'SETTING MISSED APCH TO 3000 FT.' AS THE RADIO WAS BUSY; I DID NOT MENTION THAT THIS WAS EARLY AS WE HAD NOT YET CAPTURED THE GS. SEVERAL SECONDS LATER; THE AUTOPLT COMMENCED A STEEP PITCH UP UNCOMMANDED CLB WITH AUTOTHRUST AT CLB. AS I DISCONNECTED AUTOPLT/AUTOTHRUST; I CHKED THE FMA TO SEE IF OPEN CLB WAS ENUNCIATED. INDICATING THAT THE FO MAY HAVE INADVERTENTLY PULLED THE ALT KNOB ON THE FCU; AND IT WAS NOT. AFTER GS INTERCEPT; THE REMAINDER OF THE MANUALLY FLOWN APCH AND LNDG WAS NORMAL. AS THE FMA DID NOT CHANGE TO ALT; INDICATING IT WAS LEVELING AT 3000 FT; IT APPEARED TO US THAT THE AUTOPLT MAY HAVE BEEN RESPONDING TO A MOMENTARY DIP IN THE GS SIGNAL (ALTHOUGH WE DID NOT SEE ONE) WITH AN AGGRESSIVE 'FLY UP' COMMAND TO CAPTURE THE GS.