Apple has released a statement admitting: "Some earlier PowerBook G4 computers that have a SuperDrive with a 2X DVD-R write speed may not be able to burn 8X DVD-R media reliably. Because of this, you should only use 2X or 4X DVD-R media." As Softpedia notes in its coverage of the situation, this issue has been an ongoing one for several months.
The Web site also points out that Apple has done nothing to fix the problem, other than release an advisory statement. SuperDrives with UJ-816 in their model numbers are most likely to be affected by this performance issue.