It may appear to be an Xorg or lightdm/gdm/mdm error but in reality for many users with this issue, it's a driver conflict and issue. I had a system that had two GPUs, an Intel and Nvidia GPU. The only thing that got it working was to remove the nouveau driver and blacklist it so it never came back, then the Intel GPU works fine without these issues. Solution sudo rmmod nouveau add nouveau/other driver to blacklist edit th........

You need to disable vsync like this when running glxgears: vblank_mode=0 glxgears For Nvidia drivers do this: __GL_SYNC_TO_VBLANK=0 glxgears Notice the higher than 59-60 fps results with vblank_mode=0: ATTENTION: default value of option vblank_mode overridden by environment. 7919 frames in 5.0 seconds = 1583.704 FPS 8187 frames in 5.0 seconds = 1637.266 FPS........

This is what I call a ridiculous bug in compiz that causes it. The solution/To fix it just install "compizconfig-settings-manager" Open it up and Navigate to General Options -> Display Settings Set the "Refresh Rate" to the current refresh rate of your monitor. For ATI cards with slow window dragging/response supposedly disabling "Sync to VBlank" fixes that but otherwise leave it as is.........