Solidsnake is RIGHT !!!
Simon,
As per Solidsnake post, the IMU is mounted with the Gyro’s axle along the vertical, so it can not detect when the robot is tilting. This could be the reason not being so stable as the accelerometer ONLY is being used.
If you mount the IMU board VERTICAL (and perhaps you need to change the accelerometer axle you are using) ; your robot should be much more stable.
Thanks , for sharing. Your work, as well as your research for the components is commendable.