If you're using TTG02 (TDK 8x DVD-R) ot TTH02 (TDK 16x DVD-R), I've experienced quality control problems with both types. One batch of TTG02 had some discs which burned with good quality, and the rest which were coasters. Another batch did not produce coasters, but burn quality was nowhere as good as the first batch.
I received TTH02 discs with numerous little scratches all over. This happened to about 5-7 discs of a 25pc cake box. I had a few coasters as a result and some burns that were not that good.
These are both Taiwan made versions for both types of discs.
So this is I'm agreeing with chef that you should try some other decent media (e.g. Verbatim 8x DVD-R - any type but pearl tops). Also, do follow his suggestion and post a log from Nero. That can help us figure out more of what is going on.