I am working on a fairly dense model.  When zoomed in for a mate may it be off of planes, surfaces, etc. the screen sometimes takes a zoom of the mouse ball to refresh at all.  This is intermitent and does not seem to happen consistently.

In the same model, after editing mates the graphics dissapear (with the exception of sketches) and a rebuild is required to see the graphics again.  This model is heavy on structural members.  Could this be a performance issue or a settings issue?