I don't have a NEC drive, but if your burning a disk faster directly than from your hard drive then it is probably a sytem problem instead of a problem with your drive.
But here is what I would look for (in no particular order):
1) Is the ISO your using an MDS File? Non-native images may have a chance of causing problems.
2) Ask the drive manufactuer if they are aware of this issue.
3) Go to www.alcohol-soft.com and ask the developers if they are aware of the issue.
4) Check for spyware using adaware or spybot.
5) Check and see if your hard drive is fragmented. Then defrag it if necessary.
6) Free up some resources. Shutdown programs that are running in the background and taking up memory.
I hope this helps.