When the Airbus goes into Altitude Capture mode, it flies a computer controlled arc, based on the vertical speed at the time Alt Cap engaged. Unfortunately, the A320's climb above FL300 becomes a series of roller coaster rides, as it is trying to control TAS, but doesn't have enough elevator authority (0.1G) to follow it accurately. As a result it always enters Alt Cap mode during a vertical swoop, and then the speed drops alarmingly as it tries to follow its predetermined arc (with no airspeed input to the autopilot).
Basically, this is what happened in the Toulouse crash; the aircraft was trying to follow its predetermined arc without enough power to achieve it.
Boeings use the same technique, but as their autopilots have better elevator authority and they have a greater power reserve, it rarely causes problems.