A319 Captain experiences unexpected auto thrust command for climb thrust during approach. Auto thrust is disconnected and speed stabilized. When auto thrust is re-engaged as a test; the results are the same.

Narrative

We were on a visual approach and the FMCG 'approach perf' page had been 'activated and confirmed' the speed bug was set at 170kts. I disconnected the A/P and left the A/T on. I was at flaps 1 and we were about 4 miles from the outer marker. I was looking outside to orient myself with the location of the traffic ahead on the same approach. I took my hands off of the throttles to adjust the volume of my headset. All of the sudden the engines spooled up and I looked at the airspeed and it was increasing rapidly. At the same time the First Officer reached over an slammed the throttles to idle. Being in a descent and the throttles going to full power; the airspeed was rapidly increasing to near the barber pole. We did NOT over speed the flaps. In manual; the throttles worked as advertised. I wondered if that was a intermittent glitch. I pushed the A/T button and advanced the throttles to CL detent to see what would happen. Immediately the TLA's went to max and the engines started to spool up. I immediately disconnected the A/T's and flew the rest of the approach in manual throttles with no further problems. A Mechanic met us at the gate and we explained the situation. I have never seen the A/T's do this before. The wind at the time was approx. 020@12kts