Photoshop :: Brush Icon Not Displaying Correctly

I just installed CS4 on a lovely new Dell notebook with WUXGA display (1920x1200) and am having a problem that I have never seen before. When the brush tip is small (up to 35 px at 100%)  I see the entire circle defining it, but as I move into larger and larger tip sizes the circle starts to disappear from the bottom right, becoming just a smaller and smaller circular fragment until it disappears entirely at around 300 px. If I select the center crosshair, this also disappears around 200px (when only a small circular crescent is left), and then reappears at around 400px (minus any trace of the circle). It is obviously some sort of display issue: if I change the image size the behavior is the same when the circle is the same size onscreen, so it is the display size and not the absolute pixel size which determines it. This happens whether I choose normal or full-size tip. Has anyone experienced this? It is not only annoying, it is positively serious, since at certain sizes I cannot even tell where or how large the brush is. Clear type on or off makes no difference. I have not noticed any odd display behavior with any other program.