Attempt to Burn Failed and Defective Files

There are 3 possibilities: 1) your optical drive has been slowly failing, 2) the laser lens has been getting progressively dirtier, 3) your media is failing. In your shoes (and I have been) I'd get a DVD/CD cleaning kit and a small box of new discs. Clean the lens and try one of the new discs. If you get a successful burn try your old media. If you get a fail you might want to try another round of testing before concluding that your old media is dead. Optical media dyes degenerate over time with cheap media lasting a year or so. If the cleaner and new media don't result in a successful burn it is time to replace the optical drive. If the computer warranty has lapsed you might consider getting an external burner - it will be about 25-35% as expensive as a new internal SuperDrive.