I have been setting up a new hex using APM 2.5 and 2.9.1b.  

When the GPS is disconnected and the copter is sitting on level ground the artificial horizon appears stable.

When the GPS is connected and there is a poor lock such as in a building the horizon is quite unstable showing pitch and roll movements when on the ground and completely still.  The Raw sensor data shows reflects the stationary state. The GPS location data shows significant postional variation.

When the GPS has a good lock the horizon is quite stable but not a much s with no GPS at all.

Is this a side affect of some limitation in the sensor fusion algorithm or a bug?

I have erased and reloaded firmware with the same results.