There's a bug in iDVD regarding modified assets. It's best to start over with a new project. Then follow this workflow to help assure the best qualty video DVD:
Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.
To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.
Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.
Do you know that when you have the movie as you want it in iMovie 6 all you need to do is close iMovie and drag the iMovie project file into the iDVD window to get the movie into iDVD? There's no exporting needed.
OT