Are Photoshop and OS fully updated and have you performed the usual trouble-shooting routines (trashing prefs by keeping command-alt-shift/ctrl-alt-shift pressed while starting Photoshop, 3rd party plug-ins deactivation, system maintenance etc.)?
Does turning off OpenGL in the Performance Preferences and restarting Photoshop have any bearing on the issue?
I would be dollars to donuts this is a graphics driver issue. Determine what video card you have and go directly to the manufacturers website (nVidia or ATI/AMD) and download the latest driver. GPU and OpenGL support | Photoshop CS4, CS5: http://adobe.ly/OpenGL