First of all try to try some other media. Maybe the 100 bulk DVD-Rs you got just do not work with your DVD player. Have a look here for a small list of suggested media.
If this does not work try changing between DVD-R and DVD+R if you burner supports dual format. You can also have a look at a very nice list of standalones that Videohelp has here:
http://www.videohelp.com/dvdplayers/ and try to find what type of media yours like.
Next thing you can try is burning in lower speed. Still no joy? Try changing burning program (CopyToDVD is a good alternative) and of course the movie you are trying to burn.
