It appears as if you are having connectivity problem with your ISP..
Try checking it with ping command..
Ping to your ISP server by..
Code:
ping <Ip of ISP server> -t
If there are big delays + regular breakage then its the reason you are unable to download..
You are able to surf properly because while surfing data packets are too small that you are not affected apparently in surfing but in download they are big as well as continues .. !
If its not connectivity problem .. then might your internet is getting slower than earlier.