Did you install anything outside of the repositories? It looks that it may
be the case and that it is causing issues.

When this happens I give it awhile to see if it clears up. Then if it
doesn't I completely remove (purge) the offending package, clean the cache
(otherwise it installs the package on your computer which may be the source
of the problem without downloading a new one) and re-install.

I think I have been down this path too. How do you 'clean the cache'?

sudo apt-get clean

This removes the downloaded packages stored locally on your machine.