Artifacting!

Hello, I use convertxtodvd to convert all my avi files onto dvd. I normally burn with nero at 2x speed. BUT, I’ve noticed that everytime I burn a movie ( original avi file around 800 megs on average) it artifacts during the really dark scenes in the movie. You can see little blocks and the colors get really gloomy. The original avi file is most of the time, high quality. Yet, it still artifacts when I play them on BOTH my standalone DVD players. I’ve tried using various media, the results don’t change (currently using Imation 4x DVD+RW media, only media I have available). The encoding option in ConvertXToDVD is set to high, so thats not a problem. please help, thanks!