Try using different download Server:
Originally Posted by csc2ya
System->Administration->Software Sources, under the "Ubuntu Software" tab, make sure that Main, Universe, Restricted, and Multiverse are CHECKED. "Download From:" should equal to "Main Server". And, make sure that the option "Installeable from CDROM-DVD" is unchecked. Click on Close buttong and select/click on "Reload" button on the next window that displays.
When done, Drop to your terminal and issue the command:
sudo apt-get update && sudo apt-get upgrade