Thank you for your reply, El Supremo, but that cheatcode "knoppix no3d", does not work on my computers, and again, the failures are not always manifested with the same GUIs. May I add also that to the best of my memory this Knoppix 5.3.1 disk worked fine in each of my computers for some good while after I first started using it, and these failures only began sometime later. This is an extremely strange bug, and because it affects the latest Sidux disk I have to wonder if might not be one of the problems with which the Debian people are wrestling right now.