The demo seems to run fine to me. I linked it against the latest snapshot of the source control, it should be no different to 2.1.1
Are you thinking about when the circles hit the border and stay in it? that is called tunneling and happens when the circles moves too fast. It is also described in our manual and several places in the forum. Search for tunneling and you might find a solution
that works for you.
In Farseer Physics 3.0 we will have a system in place that prevents tunneling. Just to let you know.
Also, I noticed that you are adding the geometries and bodies to the engine twice. line 139 and 140 in Game1.cs can be removed. You add the geometries and bodies on line 71 and 73. CreateCircleBody() and CreateCircleGeom() takes in a physics simulator and
automatically adds them to the engine.