Please also search the Photoshop Macintosh Forum, similar things have come up, for example:
Other than that are Photoshop and OS fully updated and have you performed the usual trouble-shooting routines (trashing prefs by keeping command-alt-shift pressed while starting Photoshop, 3rd party plug-ins deactivation, system maintenance like repairing permissions, purging PRAM, font validation, etc.)?
Does turning off OpenGL in the Performance Preferences and restarting Photoshop have any bearing on the issue?
Yup, I have rightzoom installed. Just upgraded to its latest version and problem is gone. Thanks for the help.