Sorry to say this, but PC DVD are made of the cheapest stuff available on the market. And they will eventually die.

- Try cleaning the lens ( with appropriate cleaner)
- clean the head mecanisim (dust is often enough to alter the motion and "freeze" the disk.

- DVD is a harder beast to read and convert than CD. so misalignment, dust, etc. have a bigger impact.

Other than that and the obvious facts about config and drivers, it may be time to get yourself a xmast present.

good luck.