You can't reflash the boot.img in recovery, you have to do it manually via fastboot. Because CWM or TWRP can't write a boot.img, when you restore a backup, you have to reflash the boot.img of the backup using fastboot commands.
Are you sure the ROM download isn't corrupt somehow? Have you tried downloading the zip again and trying with the new download?
Also, can you link me to the thread you've downloaded from please?