^@^@^@^@^@^@^ on black screen and then crash?

It seems random but mostly when I tried to launch something or open a new thing, the screen becomes black and show something like


for a second and the the login screen appears. Of course, if I log in, all the previously running applications are gone.

Has anyone experienced this before? Why does it happening, lack of system RAM?

Your display server crashed.

If you would be so kind to post your journal entries, that would be great and, I suspect, even productive.