Hi Patrick, Thanks.
I tried your very reasonable and nice advice. kdenlive still froze after upgrading everything, moving configuration files, and restarting. Here's the big but. BUT, I think you may have correctly guessed a problem with 3d functions. I tried running the 3d program named "glxgears". It froze too! Coincidentally, both kdenlive and glxgears a.) sometimes work for a few seconds before freezing, and b.) depend on the package named "libgl1-mesa-glx", which contains OpenGL code. What do you think? Thanks, Kingsley -- Time is the fire in which we all burn.