Large downloads need a download manager that resumes properly, browsers do not resume nor it seems does N3V online downloader. It's more than likely a problem between your ISP and the routing to N3V's servers. Only takes a millisecond disconnect which although not detectable in normal use is enough to corrupt a download, download managers that resume should be able to avoid that most of the time.
It's very easy to blame the site you are downloading from when the problem could be with any of the several Networks and Servers the data passes through, none of which have anything to do with the transmitting site or the receiving ISP.
You can often improve things by changing your DNS servers from the ISP's to OpenDNS or Googles DNS.