Now I'm experiencing spontaneous resets of the desktop environment (when logged in and working normally), where the screen will go black, then reset to the login screen.

• The resets appear to be random, though there is a chance it occurs more when using Nautilus.
• The screen goes black instantly, e.g. there is nothing like a disappearing launcher or window borders.

Where can I begin finding the cause?

(Note: I've recently upgraded from 11.04 to 11.10, then 12.04.)