Plane turns inverted in FBWA

I have set up a new plane with the APM mounted upside down. It is also mounted at a slight angle. I have done the orientation and the HUD shows correctly after I did the 3 axis calibration.

Switching to FBWA on the ground, everything works as expected. However once I fly and switch from Manual To FBWA the plane rolls in a random direction and if left for long enough, it will actually try and fly inverted with a heavy oscillation. The plane is not tuned in any way yet and I am not using an Airspeed sensor.

Firmware that is used is 2.78 with a APM 2.5.
No Airspeed sensor.

The logs are too big to post here. I have a smaller log at the office and will post that tomorrow.
Any help appreciated

Hi Andreas,
a log would certainly be helpful in diagnosing the problem
Cheers, Tridge