The viewing area of the monitor screen keeps shrinking
down to where I can only see about a 2-inch-square area.  
The rest of the area around it turns black.  Sometimes
the sides of the viewing area turn black in a concave
fashion. When the viewing area shrinks, the writing gets
so small I can barely read it.  I used to be able to fix
it by pressing the degauss button, but that doesn't do
anything anymore.  Sometimes it returns to normal after
10 or 15 minutes, without my doing anything.  Any ideas?