Today I had updated by apt-get update and received a message like this:
W: GPG error: http://nz.archive.ubuntu.com breezy-updates Release: The following
signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key
<ftpmaster@ubuntu.com>W: You may want to run apt-get update to correct these problems…..
I have searched at launchpad and found this bug , and old one but still alive. So for fixed I ran :
$ sudo apt-get update -o Acquire::http::No-Cache=true
and that’s all. I guess I heard about this was a problem with Round Robin DNS (Maybe), anyway doing this you can solved the problem.
PD: I forgot say that happen in Ubuntu Intrepid 
