desktop crashing

sorry to be such a bother, but as long as i'm bringing things up, here's one that has been an issue for a couple of years and survived into R14. my desktop crashes, but in such a way as to not even be noticeable at first. i have in lieu of wallpaper xplanet set up as a realtime moon phase indicator, updating hourly. the way i tell the desktop has crashed is that it doesn't update -- in this case it last refreshed at 10:30 a.m. yesterday. i believe that i was probably editing pictures at that time, though would not swear to it. after it crashes, a right click on the desktop does not produce the expected menu. as it happens, kicker and everything else continues to function as expected -- in fact, it was just now that i noticed that the desktop itself had gone south. i thought i'd look in the x error log, bit i see that the one i have, ~/.xsession-errors, is of an unspecified file type and won't open in a text editor (it reports its size as 500.0 k). any idea if there's a log that might let me see what's going on here and if so what log it might be? bonus points, how i might restart the desktop without logging out and back in? extra bonus points, how i might fix it so it doesn't do this anymore? i'm running some flavor of ubuntu 12.04LTS -- i say some flavor because when i sought to upgrade a few days ago the ubuntu upgrader refused, citing third-party apps or a beta version of the opsys, or something; it didn't specify. i thought i had bog standard 12.04LTS on the machine but am apparently wrong. -- dep The shortest distance between you and playing great acoustic guitar: the great new instructional DVDs from Marjorie Thompson, available at www.MarjorieThompson.com