Are Photoshop and OS fully updated?
As with all unexplainable Photoshop-problems you might try trashing the prefs (after making sure all customized presets like Actions, Patterns, Brushes etc. have been saved and making a note of the Preferences you’ve changed) by keeping command-alt-shift pressed on starting the program or starting from a new user-account.
System Maintenance (repairing permissions, purging PRAM, running cron-scripts, cleaning caches, etc.) might also be beneficial, Onyx has been recommended for such tasks.
Weeding out bad fonts never seems to be a bad idea, either. (Validate your fonts in Font Book and remove the bad ones.)
If 3rd party plug-ins are installed try disabling them to verify if one of those may be responsible for the problem.
Does turning off OpenGL in the Performance Preferences and restarting Photoshop have any bearing on the issue?