The reason why the Aero sometimes is flying inverted is due to a change on the parameter “INVERTEDFLT_CH” and accidentally triggering the customized channel.
To disable:
1.- Please access the Full Parameter List on your Ground Control Software and look for this next Parameter “INVERTEDFLT_CH”. The description for this parameter is:
A RC input channel number to enable inverted flight. If this is non-zero then the APM will monitor the corresponding RC input channel and will enable inverted flight when the channel goes above 1750.
2.- Set it to 0. It has been disabled.
From <http://plane.ardupilot.com/wiki/arduplane-parameters/#Inverted_flight_channel_ArduPlaneINVERTEDFLT_CH>