Binary package hint: kdebase-workspace
After an upgrade to Natty stable, I found that the stability of the graphical system degraded significantly when using KWin with the NVIDIA proprietary driver (tested with nvidia-current 270.30 to 270.41.06 on a GeForce GT 240).
Graphical artifacts (colored dots, aka snow) appear when moving windows or playing videos with desktop effects enabled. When they are disabled, different kinds of artifacts, along with flicker, appear on window borders instead. In addition, in both modes, opening or resizing a window sometimes causes the entire system to freeze.
If these artifacts are present, switching to another window manager (like Compiz) does not fix them, but switching the window manager in KDE settings and rebooting does. I could not reproduce this bug when booting with Metacity or Compiz, or with KWin under Nouveau 2D or Gallium.
The bug seems to have gone away in KWin after I unset the "Use VSync" checkbox in desktop effects settings and rebooted.
I must say I have the issue. Whenever I write something in a text box in the browser (like right now) all kind of weird dots appears on the screen.