If I ever have that, it's usually because of something with a memory leak, or desktop graphic compositing is degrading over time.
Before you reboot, see if you can ssh into it from another computer and check htop (or just top all you have), see if something is consuming all your memory or cpu.
Desktop compositors are all shite from what I can tell (including windoze), they're all just a varying degree of broken when you push them, or just have a slow (or buggy) video card. Depending on your desktop environment of choice, try turning down graphics options, or disabling the compositor all together to see if it helps stability.
-mb