The Synaptic Manager has a built in option for a proxy server to speed up downloads and manage them.
On the other hand, 'sudo' and 'apt-get' uses direct HTTP connection to get the files.
My suggestion is load up the Synaptic Manager and go through the options, look for the PROXY server options and completely disable them.
Using a proxy to download packages might some time cause the remote repository to mis-identify your correct version of your distro.
I am using Ubuntu 8.04 x64 version.
And I have encountered that problem myself, I just disabled the proxy option and waited for the download to complete.
I prefer this option, even if the download might be slow I want the correct package for my distro.
I hope I helped out,
Have a great day.