And the version of the OS you are using is a secret why?
Boilerplate-text:
Does turning off »Use Graphics Processor« in the Performance Preferences and restarting Photoshop have any bearing on the issue?
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.
http://www.apple.com/downloads/macosx/system_disk_utilities/onyx.html
Weeding out bad fonts never seems to be a bad idea, either. (Validate your fonts in Font Book and remove the bad ones.)
http://helpx.adobe.com/photoshop/kb/troubleshoot-fonts-photoshop-cs5.h tml
If 3rd party plug-ins are installed try disabling them to verify if one of those may be responsible for the problem.
Hi and thanks
Mac version is 10.7.4
The graphics processor idea didn't work and i can't see any tool called Onyx on that link you shared. I did find this http://www.onyxmac.com/ but it looks a bit suspect to me. To the best of my knowledge i am not using any plugins.
Thanks
North America
Europe, Middle East and Africa
Asia Pacific