I have Ubuntu 13.10 x64 installed as a guest in VirtualBox (under a Windows 8.1 host) and have the settings for the virtual machine setup to run with a monitor count of 2, 128MB video memory and 3D acceleration enabled. In my guest I have the virtual box additions installed (which allowed me to have two 1920x1080 screens). Here's a screenshot of my VM settings. My laptop is an Asus N550JV which has both Intel's HD Graphics 4600 GPU and Nvidia's GeForce GT 750M. By default though I believe the Intel GFX card is being used to render the VM. When I boot up the VM it loads perfectly on dual screens, however whenever I move the mouse from one screen to the other (I have a Dell S2340L running over a HDMI connection as a second screen) the screen flickers. I've tried a variety of settings changes in both Ubuntu and the VM settings, but cannot seem to stop this screen flicker. I also used the NVidia control panel in Windows to force the dedicated graphics card to always be used but found that the display driver sometimes crashed whilst working in the VM, resulting in my VM session being destroyed, so I figured it's better to stick with the Intel GFX as that appears to be more stable. I also tried without 3D acceleration but that was much worse, and if I ran the VM with a low amount of graphics memory it really struggled. Here's my dmesg output: http://pastebin.com/1LJuYWMj (not sure if this is helpful in this situation). I read some posts suggesting changes to /etc/X11/xorg.conf but I don't appear to have an xorg.conf file. There were also a few posts (though related to Synergy) suggesting running xset -dpms but this command doesn't appear to have had any effect for me. As an additional note, I'm finding that window drawing in the guest is a little laggy/glitchy. For example, quickly scrolling through a web page may result in parts of the viewport displaying original content. Certainly I notice drawing issues most in the web browser, but it also impacts other software with parts of the window not being drawn when, say, switching between accounts in thunderbird. Any suggestions greatly appreciated!