Yep. I had similar problems a few times in the past; I never could get any real insight on what was causing it or help fixing it. I tried so many things, I can't really recall what resolved it, but I'll look through my notes and see if they can jog my memory.
But, in the end, I stopped running Slim and started logging in to a console and running startx. Primitive, but it always seems to work.
Tim
Later - My notes contain a lot of mess. Some of it involved moving nm-applet startup from /etc/xdg to LXDE startup. But I later decided that that had not really worked. Then it seemed to disappear after a kernel upgrade (3.11.2 to 3.11.3). Later on, I disabled network-manager entirely and startted configuring static IP's with ceni. Take all of this with a grain of salt, because I still have no idea what the real problem was or why any of this may have fixed it. But, it hasn't happened to me for the past couple of months.