Welcome to the forum
Could be the software/conversion process or a media problem, I'll tackle the media
What's the rated speed of the discs you're using, and also the brand?
What burner(s) are you using to burn the discs, and what actual burn speed are you choosing?
EDIT: Another possibility, if your burner allows it, would be to set the booktype in Nero to DVD-ROM (since you're using +R discs), for improved compatability on older players.