Xrox, I have tried Generic Media (BulkPaq - Princo) as well. Bought 50 disc in box's of 25, and the one box burned 100% perfect and then I opened the next box and started out with 11 coasters and suddenly the last 14 disc where ok, so its probaly the media rather than the Hardware. Have you tried Scanning them with K-probe?
Have you burned any data (Other than Xbox games) on then or any movie's, when you burn dvd's do you verify your burn?, most program can do that.
And if you use DVD+R Media, I think I have read that the xbox can't read Book-type DVD+R, so you have to change it to DVD-ROM. Think you can DL the utility from www.Liteonit.com or in the Utility Section on this fine site..