As has already been explained, the most likely cause of your problems is that you are running out of space in ram for the temporary iso files. As has already been suggested, you could choose a location for the temporary files on permanent storage to solve the problem.
It is not obvious why you are choosing to remaster an iso and boot from it - why not use a usb stick, it would easier and quicker?
Edit: As a test, I took the CorePure64-10.0 iso, added Xorg-7.7, flwm, wbar and aterm onboot outside the initrd and made a new iso with ezremaster - the new iso booted with qemu without problems.