AutoQuad uses a barometer (air pressure sensor) together with the accellerometer and GPS (if available) to maintain a steady altitude in position or altitude hold mode.
But electonic barometers are sensitive to light, temperature and wind gusts.
Regardless of if you are using the Analog IMU (AIMU) or the digital IMU (DIMU), you need to shield the barometer sensor from sunlight and sudden gusts.
The most common method is to use some open cell foam. Basically any kind of dark foam that you can blow air through without significant resistance will work. Pick´n´pluck foam and sounddampening foam usually works well.
For the DIMU addon board, the simplest method is to cut a small piece of foam and sandwich it between the DIMU and the AQ6 board like shown below. The DIMU barometer is the small chip with a metal lid on the underside of the DIMU Addon, so make sure that chip is covered the foam. The foam should just be large enough to fit without any significant compression when the DIMU is mounted:
For the AIMU, you basically need to cover the barometer with foam. This can be done by inserting a piece of foam between the AutoQuad flight controller board and the GPS groundplane shield.
Cut a little square pocket in the foam with a small scissor and fit it over the analog pressure sensor: