Another help source (if you haven't already done so).
Go to your OS Help Menu. In the search field type dvd or cd
Click on all the troubleshooting topics & Support Articles that pertain to your issue.
You can also do the same in Disk Utility. Open same up.
At the bottom left of the window, click on the purple button w/the "?" in the middle.
This will bring up the Help Menu.
Try a different brand. Top forum favorites
Make sure the DVDs are not dirty, smudged and/or scratched.
http://docs.info.apple.com/article.html?artnum=50448 How to Handle and Clean CD and DVD Discs
Your drive may need cleaning. Cleaning kits can be purchased from any store that sells CD/DVDs.