The reason is two-fold, knowing the age of the computer, I can find out the Superdrive has the following support:
"SuperDrive (DVD-R/CD-RW); writes DVD-R discs at 1x speed, reads DVDs at 6x speed, writes CD-R discs at 8x speed, writes CD-RW discs at 4x speed, reads CDs at 24x speed"
This also tells me whether or not a firmware update is needed for the Superdrive:
http://docs.info.apple.com/article.html?artnum=86130
Which as it happens: "The following SuperDrive-equipped computers include the latest update. They can read from and write to high-speed media. (No update is needed):
iMac (17-inch Flat Panel)
eMac with SuperDrive
Power Mac G4 (Mirrored Drive Doors) with SuperDrive
PowerBook G4 (1GHz/867MHz) with SuperDrive" [my emphasis]
So it would appear if the problem is there, it is more than like related to the media, the burn speed selected, or some bad preference or cache file out there. Now as for the media, these media types have the highest reliability:
1. Verbatim, Kodak, Imation, and Sony for CD-R
2. Burning at below the highest supported speed for the type of media your Superdrive supports.
3. Not using labels or writing on media, and buying jewelcase instead of spindled media.
So the question is, is your media meeting all those qualifications?