There's been some debate about this, but, if you've time, take one of the offending discs to your local electronics retailer and try it in one of the new DVD players that specificially support DVD +r/-r/rw media (most current models do). It'll probably work.

Some time ago, I had the same problem with an older Samsung player--backups either failed to start or would break up and stop during play. Tried a number of different media, burned at different speeds, different burners, etc. None of the backups played on the Samsung, yet the original discs (as well as rented movies) all played just fine. I had similar problems when I tried to play the backups on friends' older (2+ years) DVD players. After I replaced the Samsung with a new Samsung (which supports the newer media), the same discs played perfectly and continue to do so to this day (they also play in a new Sony player I have in a second system).