A buffer underrun occurs when the PC cannot deliver data fast enough to the drive’s buffer. If the drive’s buffer empties because of the lack of data, the laser will turn off. The resulting gap on the media usually renders a CD-R disc unusable. A CD-RW disc can be erased and re-recorded in this case.
Hmm...doesn't your burner have buffer underrun protection? I thought that was standard these days...old burner, perhaps?
Read
here about your problem.