I have not experienced this behavior with PSCS3 and Leopard. The first thing that you should do when experiencing erratic problems in Photoshop is to reset the preferences.
Hold down Alt, Ctrl, and Shift keys (Mac: Command, Option, Shift) while starting Photoshop. A dialog box will appear asking if you wish to delete the preferences file. After the preference file is deleted, Photoshop resets to factory defaults.