The 'Out of Memory' message is due to the rootfs filling up. This can happen if you are installing applications from extras-testing that take large amounts of space and aren't optified.

the root partition can also fill up if a bunch of apps are quickly installed and uninstalled. there is a BZ tracking that issue already. Rebooting seems to be the workaround at the moment.

I have experienced OS sluggishness a couple of times in the past 4 weeks. I think this happens because of apps leaking memory and if I haven't rebooted in a while.
I wonder why apps like modest, camera-ui, browserd each use about 20mb of memory even when they are not open.

Anywya, Xorg runs with a log file in /tmp/Xorg.0.log. It may provide helpful information when reporting bugs related to UI slowdown.