Downloads don't finish

system: Mac OSX 10.8.2

Dear All,

I have an annoying issue. I have had this for some time now. If I am trying to download an application (.dmg file) outside of the App store, it stops when I have 100% of the download without converting the file to a .dmg file. It remains as being a .download file. I called the apple helpline and they said it is the ISP who is to blame, called them and they blamed Apple. Is there a way to solve this? Applications that I am trying to download include firefox, skype and others.

I reinstalled my mac, but the issue persists.

Other sources are working fine (itunes store, app store, etc). I have set the gatekeeper to accept downloads from any source (not just app store). Would be great to get a solution.