Ubuntu Lucid logs me off

I leave Ubuntu running with some applications and leave the computer alone by itself without using it, If I pass by I can see the screen saver is running. I go out the house or go to sleep, when I come back, the login screen is what shows up. Something is crashing and logging me out. I know my computer is not restarting because my BIOS pauses and shows a menu from which device to boot. I don't see this menu when I come back, I just see the login screen. I running Ubuntu 10.04.

Note #1: I went to #ubuntu-bugs at freenode and they asked me if I could see some errors on the document ".xsession-errors.old". I saw some IO errors that may hint what's the problem. Here's the part of it: http://paste.ubuntu.com/434109/plain/

Note #2: I have the Kernel Mode Setting disabled so Ubuntu be able to work with my monitor.

Thanks in advance for any help.