What may cause the desktop wallpaper to be scrambled?

I have a very strange problem. Sometimes when I work a long time with my laptop while the desktop is hidden a strange thing happens. I show the desktop and it is all scrambled.

This problem is all the stranger since it happens only sometimes and I did not have it always.

How can I diagnose whether it is a hardware or software problem? Has anyone had similar symptoms?

EDIT:

This problem persists after upgrading to 13.10.

When the wallpaper gets scrambled and I change it to a different one and then back again, it seems fixed for a while. Does this fact indicate that it is not a hardware problem?

EDIT2:

Now this happens also for some windows, not just the desktop.