Yes, it's Ubuntu, and I THINK I've have just nailed the problem. The idea that it might be a video driver problem led me to it.

Briefly, Ubuntu desktop has a bunch of fancy special effects, and this is accomplished by a 'compositing layer', a program called compiz, and all video output works through it - including the virtual video of the guest vmware machines. Sure enough, turning off special effects in linux (which turns off the compositing layer) seems to have cured the problem.


I am very hopeful the problem is now gone