Bit of an nuisance this one and it only crept up in the last few weeks. Turned out to be FileZilla which was the culprit.
From the login screen switch to a terminal
ctrl + alt + F2
Login with your usual credentials
Run top and look for FileZilla
Copy the PID from the first column in top and kill it!
sudo kill PID
Switch back to your login screen.
ctrl + alt + F7
It is more than possible that other applications could be causing you the same issue, it just happened to be FileZilla for me. Looking at the processes in top, you should be able to get a good idea of what might be causing your problem.