Hi Dave, On a Windows system this is generally the sign of incompatible OpenGL support from the video card/driver. We usually test it by turning off hardware acceleration under Preferences (this is in the SketchUp menu on the Mac as I believe) > OpenGL settings. Now I understand that Macs work differently and your hardware support (video driver) is included in the OS (or something) and maybe you do not even have this option (?) Anyway, if you do, can you check if it seems to "fix" it?