That computer kicks mines in the bu...
Have you tried DMA configuration?
Perhaps DMA is off in NEC or even DMA config at BIOS is not correct.
I think it should be set to "auto" .
I think transfer rate from hard disk may be wrong, but using a Raptor... it looks hard to believe. Anyway if there is a buffer underrun, I guess NEC (as any new CD recorder) has it's own buffer underrun protection so the CD won't be a coaster unless this protection (burn proof , burn link and so) is off.
HAve you tried VIA 4 in 1 (or the new VIA drivers. I don't remember the new name) ?
IT solves DMA and IDE related problems...