X server uses 100% CPU and freezes at user switch

When switching user on fedora 18+KDE, the X server starts using 100% of CPU and the screen freezes.
This happens very often. It seems that this happens more frequently after one of my chilren played with minecraft.
I have added -logverbose 6 to my X server command line and have run nvidia-bug-report.sh.
I would like to attach the nvidia-bug-report.log.gz file but I do not see any button to attach a document. Can you help for that please?
nvidia-bug-report.log.gz (142 KB)

Now that I have posted my message, I can see the button “Add Attachment” but when I click on it and select the file, nothing happens.

OK, I know what happened: the file was created as root so root only was able to read it… chown did the trick and I have now posted the bug report. Hope this will help.

New crash when shutting down the computer. Black screen in text mode showing Fedora 18 at the bottom and CPU at 100% for X server (I was able to go to a new text windows with Ctrl+Alt+F2 and log as root to build the bug report). Then, reboot hanged: I had to manually power off the computer.
nvidia-bug-report.log.gz (180 KB)

New crash while leaving my session. The screen freezes on the desktop background. I had to manually turn off the computer. I ran nvidia-bug-report.sh only after turning the computer on again. Is it useful?
nvidia-bug-report.log.gz (162 KB)