Dvd burning problem with the drive.?

This is what happened during last few days. 3 dvd's failed at 98% of the burning process. (burned part was readable). Then i reduced the burning speed to 8x. Dvd's burned ok. Yesterday i burned a dvd with 12x speed. It finished successfully. But when i re inserted it, it showed a blank dvd. The face of the dvd looked like of a burned one. (i mean it had those circles). Dvd drive is about 2years old. And had burned about 120 dvds. It is able to play dvd. What is seeming the problem here? I'm using nero 12.