I have this problem as well using nvidia 177 + xinerama. I'm trying out the twinview fix (kdesudo nvidia-settings => display configuration => configuration => configure => change from xinerama to twinview) with no problems so far. Using twinview, performance is much better and kde4 transparency + desktop effects are now enabled. Thanks for the suggestion!
I have this problem as well using nvidia 177 + xinerama. I'm trying out the twinview fix (kdesudo nvidia-settings => display configuration => configuration => configure => change from xinerama to twinview) with no problems so far. Using twinview, performance is much better and kde4 transparency + desktop effects are now enabled. Thanks for the suggestion!