One question... can you get a copy of your PHP.ini file? It's probably a server issue - not a DFP issue. I am willing to bet your server either is "timing out" as you are having a PHP script download the file rather than FTP which is a secure always connected protocol. You need to see what your memory limit for processing scripts is and the maximum time allowed or you will get as you described, a partially downloaded ZIP which will not work.
Best regards,
Brian