I haven't tried your sample, but just a couple of ideas from the XAML:
- it looks like you have your "Ball" ellipse _inside_ of your "Globe" ellipse, correct? I'm pretty sure this would cause irregular physics behavior because you need to image that the shapes are solid. At least, I've never tried doing
what you're doing with FS3 :)
- you could try to turn DebugMode="true" on the PhysicsControllerBehavior to make sure the boundaries are being determined correctly (although in this case I don't think that's the problem).