[ubuntu] can't update

I've been having a lot of problems with updates recently, and none of the fixes I've found seem to work.

I get this:


Using apt-get upgrade on the command line only installed some of the updates, and none of the other things I've found works. Changing the servers actually makes it worse, as some of the updates disappear. Running apt-get update doesn't show any missing keys.

I'm using Ubuntu 12.04, in case that makes a difference.

Thanks for any help.