Please provide a full burn log, there maybe something in there that can tell us what is goin on !
Memorex media (Media ID: CMC Mag) are one of the worse media on the market.
As for the TDK DVD-R 16x, are those TTHxxx media id ?
They should be mucg better than the CMC Mag ones, but don't burn them too fast, 8x max (Personal Opinion).
I have a NEC-3550a and only burn Taiyo Yuden media, and I love it !