I have sometimes had trouble with CD's burned at high speeds or on CDRW media. Burning at 8x or less would fix it (and make sure the md5sum of the download is correct). I usually download the iso images to an NFS-shared directory on the network and only burn the first one. If you enter 'linux askmethod' at the boot prompt you can pick NFS as the install method and point it to the exported server/directory. It's faster and you don't have to stay around to swap disks. You can also boot with the floppy images if you really have trouble with the CD.
Note that it is not necessary to mount the ISOs via the loopback, the installer will find the files it needs within them.