I'm having an issue with my nvidia-331 driver. I can't seem to switch between Intel and Nvida via nvidia-settings. The option is there but when I click 'nvidia' a blank error window pops up. It's very weird because I've been using the Ubuntu daily for a while on 14.04, and it was not until yesterday's update that I've had this issue. Same driver. Does anyone know how to fix this? Thanks. I've tried a clean install and have the same issue.