What should I do when "An error has occurred" when downloading Lion? I'm trying to download the new Mac OS. When click install, the icon pops onto the dock and says Waiting...

After about 5 minutes, it disappears and in the purchased category of the Mac App Store, next to install it says:


  An error has occurred


Why is this happening?

Edit: 
Originally I could not even get the download bar in the purchased section until I reconfigured my router slightly(I'm using a BT Business Hub). I am now able to get the download bar. However, I still get "Waiting..." on the icon and after about 3 minutes, I receive the same error message:


  An error has occurred


I still believe that it is a router problem and not the app stores' servers. Has another been able to fix this yet?

Here is a screenshot when I click the install button:


And here is a screenshot after about 2 minutes:
I have sorted the issue. I just had to use a different router. The BT Business hub that I previously been using had a problem on it which prevented anything above 2GB do loose connection. This would explain other problems as well.