As a suggestion for CAU stall: malfunctioning mass flow control valve (pack valve opening rapidly) or a by-pass valve oscillating, damaged impeller blades, semi blocked heat exchangers/intercoolers or ducting thereby causing a back pressure which could also be caused by icing in a downstream water separator.On B727's a unit similar to a CAU was used as a flow multiplier when the APU supplied air con on the ground, this could stall if the system ambient air inlet was blocked or crosswinded.