I'm using normally the ubuntu, version:

~$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=13.10
DISTRIB_CODENAME=saucy
DISTRIB_DESCRIPTION="Ubuntu 13.10"

And so, for no reason, the screen goes black and come back to login screen. This not happen on login screen, like login screen infinite loop problem. I get the login and use it normally then suddenly the screen goes black. I was using Ubuntu 14 and for this problem I formatted and installed Ubuntu 13, but the problem remains.