Comment 18 for bug 1049630

I also have this problem. In my case, the corruption is not in the form of horizontal lines; simply the area inside any window that requires 3d graphics is black or frozen. I use Ubuntu 11.04. Compiz simply doesn't start, and the same behavior for programs like Stellarium or the visual module in Python (that I use very often). This behavior was **not present** a year ago in Ubuntu < 10.04. 2D acceleration works well (as a workaround, I can use `export LIBGL_ALWAYS_SOFTWARE=1`, but of course that is not a solution because graphics are extremelly slow).

The issue does not occur always. "Sometimes" I have an slow 3d acceleration (that I suspect it's really software rendering), and "sometimes" I have normal 3d acceleration (fast, smooth, as expected). I really don't understand when it works and when it doesn't work. If you give me instructions, I can help to clarify this. /var/log/Xorg.0.log has nothing strange.