Having similar problems with +R DL backups lately. Not sure whether its a media issue or burner gone bad... or something else.
I'm using a Memorex DVD16+/-DL4RW|ID2 JWS6 burner and RiData 8.5G 2.4x media. Used this setup for about a year reliably. Now, using CloneDVD2 and AnyDVD, most recent backups and repeat retries are bad, all around the chapter break (my quick test is to play the DVD a few seconds at each chapter till I glitch, freeze, or reach the end).
I'm starting to suspect bad media. Using the free Nero CD-DVD Speed tool I'm seeing lots of errors around the middle of each burn (chapter break).
One thing I'm starting to wonder about... are commercial DVD movies getting bigger, as in more data. I'm wondering if older (successful) burns were simply smaller files, that didn't hit more sensitive regions of the disc.
I'm still trying to figure this all out.