Try this:
Go to the hard drive>System Folder>Preferences. Now locate:
Apple DVD Player prefs
CD/DVD Prefs
DVD Volume DDB
DVD Volume DDF
Drag these to the trash. Close the windows, restart the computer and at the same time hold down the option and the apple keys. Continue to hold them until you get a dialog box asking if you wish to do a desktop rebuild. This will come up after the extensions load at the bottom of your screen. Now release the keys and click on okay. When that is done, empty the trash. Now give it a try.
Have you check Software Updates?
Apple menu>Control panels>Software Updates. Have it check for any updates that you may need.