Might be bad optical media. When Leopard boots, it will do a file system check if it thinks there may be a problem with the drive. Doesn't seem out of the question that optical media would be treated the same way.
I don't know if you can do this, but try option-booting the system, then right after you pick the DVD, hold Command + V. That should let you verbose boot the system, which might give you more clues as to what's causing it to restart.
Good luck!