I have been using FU 2,6 LE to make h.264 backups of concert dvds that I have. Often I get progressively worse synchronization between the audio and video as the concert plays. The offset can be corrected in VLC player but it is "hit and miss". Some DVDs come out perfectly while others are almost unwatchable due to the sync issues. What causes this and how can I prevent it?

Thanks for your help.