First create a disk image of the iDVD project
mount the disk image and play with DVD Player to separate the encoding process from the burn process.
If you can create and play a disk image then the problem is with the burning and not the encoding. Also saves some disks from being coasters.
Follow this workflow to help assure the best quality DVD video disk possible:
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 and launch DVD Player and play it. If it plays OK with DVD Player the encoding was 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.
Then consider the post by Bengt regarding encoding errors in this topic: Stage 4 Multiplexing - Media error in...: Apple Support Communities. Bengt is way more knowledgeable about encoding and such than I.