The burner's buffer will fluctuate a few times per burn because NEC uses Z-CAV, which means it divides the disc into zones, and when it gets to the next zone, it changes speed. At this point, you will see the "Buffer Level" (in Nero 6.6) fluctuate.
Please note that this is not the same thing as the "User Buffer Level" - if that fluctuates, it means your system cannot keep up. Even with the fastest hard drive, if you are burning lots of small files that are badly fragmented, it won't be able to keep up even with 8X (~11MB/s). However, since all modern drives use buffer underrun protection technology, it shouldn't affect your burns much (although you may see some spikes when scanning the discs - just ignore them).