If you can confirm that somebody else is able to download those same files from the same server, that will narrow things down a bit. The fact you're getting the exact same incomplete files each time (if I understand you correctly) suggests they're not complete on your server, in which case you'll need to use another one.

On the other hand, if they're incomplete but *differently* with each attempt, then the likely culprit is your system: bad RAM, hard drive, network card, nuisance anti-virus, firewall etc.