[ubuntu] Occasional low-graphics mode after failed Macbuntu install

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http://ubuntuforums.org – I have a dell-inspiron n5110 and I just upgraded to Ubuntu 12.10, I tried to install Macbuntu 10.10, previously I had gnome-fallback installed. The installation didn't completely work or seemed to be done, I logged out and back on and all was fine, and I uninstalled Macbuntu. Now occasionally I can log on perfectly ubuntu with full graphics (with an occasional system problem detected), but just as often, sometimes after having the successful login screen, I am forced into low-graphics mode, can't use mouse or anything, and am told I'll have to configure the graphics. I would like to know h (Hardware)