Yeah this is actually a common issue in the Farseer Physics Engine (the underlying engine used by the Physics Helper).
But there are several things you can do to make it better. Take a look at the thread below....
... and keep in mind that you can get a reference to the Physics Simulator by using a line of code like below...
PhysicsControllerMain _physicsController = LayoutRoot.GetValue(PhysicsControllerMain.PhysicsControllerProperty) as PhysicsControllerMain;
... after which you can get access to the Farseer Simulator object by:
... from there you can toy directly with Iteration, Bias Factor, and the Inactivity Controller.
Hoper this helps!