Have you tried disabling desktop effects/compiz? What graphic card driver are you using? Does it happen, if you use the free one (e.g. "nv" instead of "nvidia", if you have a nvidia card)?
Have you tried disabling desktop effects/compiz?
What graphic card driver are you using?
Does it happen, if you use the free one (e.g. "nv" instead of "nvidia", if you have a nvidia card)?