Ditto, seems this is still a well known problem
. As I recall from nvidia's past responses, this is you running out of video memory. How much ram does your card have? Me? I have 320mB, but it still runs out with a few large windows (I have a 2560x1600 screen) and I have to start kwin --replace to temporarily fix the problem. This happened to me for years on compiz and what caused me to try kde/kwin in the first place ( google black window's bug and compiz, you should get a billion hits).
One thing of importance to ask you is if you are using compositing in kwin or compiz.
@nvidia: howcome kwin --replace or compiz --replace temporarily resolves the problem? It would seem that the same amount of memory is still in use from all non kwin/compiz proceses. Is it just kwin/compiz leaking resources not tracked via normal kernel mechanisms or is this strictly a driver problem? I never see that much real (res)/virtual memory (virt) claimed within my apps change and only kwin (+plasma)/compiz shrinks when restarted as according to htop. xrestop never shows anybody to blame either (there were those plasma leaks of 6 months ago+ but they've all been plugged.. as of this moment I've only got 210 megs in xrestop on a 320 meg 8800gts and I have no problem getting a black fullscreen with mplayer and vo=xv as I type this)