FLS,
Mongoose could be mouse flying, and YES, this is an entirely new flight control system for the mouse. Choose F3 external view mode to see what I am saying. Dale does allow you to toggle back into the old control input method, but as the F3 view shows some things definitely have changed. I definitely will not say flight model, because that refers to the physics and handling of each aircraft.
Does that help? Oh, and there are a few different modes, settings and button configurations that can help you fine tune your experience, and not to mention the default field of view has changed.
It may take a bit of trial and error, but it is better for alpha testing to report these "out of the box" experiences before manually adjusting.
If you are not mouse flying and referring only to some sort of system problems, you should include your dxdiag for the staff or even other testers to look and see if there are any obvious answers to your experience.