How much RAM and spare hard disk capacity do you have?
Try resetting the Photoshop preferences:
1) Close Photoshop 2) Double click on the Photoshop icon to launch it 3) QUICKLY hold down Ctrl+Alt+Shift. (for Windows) or CTL+OPT+CMD+Shift (for Mac OS X) 4) When a dialogue appears, asking you “Delete Photoshop Preference Files?” Click Yes. If you don't see the question repeat the procedure again.
If you have the correct system specs and resetting the preferences hasn't resolved the issue, uninstall Photoshop and reinstall it again (if a previous version was working, roll back to that version until the issue with the current version is resolved).