Incomplete downloads could be caused by multiple factors such as Internet connection speed, download accelerators, malware, ISP filters, server restrictions and so on.
If you say that the download worked fine for a period of time, then there is something wrong from your side. If you are using a download accelarator, try turning it off for a while and perform a new download. Another thing could be the browser's settings. In most cases, a proxy could be the problem and you can check that by going to Network/Connectivity/Proxy Settings areas of any browser.
