Your problem might be because you are running the backup from burners. Do you have a regular DVD Rom? If not, you can pick up a Liteon DVD player for fairly cheap, and they are excellent for reading backups.
Also, you might consider upgrading the DVD burners. The "Sheep lists" are relatively old, but I think that the Nec 1100 made the list and from what I remember it was not able to do correct EFM encoding. The Nec 3500A is selling for around $60 (US) at Newegg, and it hasn't failed me yet.