There is a destinct possibility that either statusbar clock or personal menu (more likely personal menu) has a problem that is crashing the desktop. But them disappearing do not mean they are causing it.

When the desktop crashes, the tablet reloads a 'safe'set of statusbar and task navigtor plugins and disables all home applets. The statusbar applets and task navigator loads are controlled by a library set in the ~/.osso/hildon-desktop/desktop.conf file referred to by X-UI-Policy.

I have been experiencing this as well (and thought it was modest related) so I will look into personal menu.

Out of curiosity, did anyone notice this with earlier versions of personal meni?