Could be a number of things. If you are using UltraISO and replacing the edited file directly in the ISO image then I'd suspect the DVD writer or media. The MK IV DVD drive is very fussy when reading DVD+/-R disks, with some drive/media combinations it will only work when you burn at the slowest speed, with others it will only work at the highest speed, others it won't work at all. Just because you can read the DVD on your PC does not mean that the Mk IV will read it correctly. Try making a direct copy of the working DVD you have, when you can do that you have at least removed one variable from the equation.