For me the problem was IPS . I tried installing 8.1 at my place almost 10 times but the download used to get interrupt in between . Later i went to my friend's place who used different ISP and also has 2mbps download speed my 8.1 installation got finished in one go. Thats why i think ISP was the root cause since if your internet connection gets cut off even for a small time , you may experience a break in download.
This worked in my case.