[Chimera-users] saved image discoloration

I've successfully used Chimera in the past to make beautiful images for publication. However I recently installed v1.2199 for Mac OS X and now whenever I try to save an image I get discolored boxes throughout the saved image. (see attached) The position of the boxes varies slightly each time. I have the image width set to 1050 pixels, 300 pixels per unit, and supersample at 3x3. Any suggestions would be appreciated. I do have an older computer, is this possibly a hardware issue? Megan