LENOVO ThinkPad T420 running 12.04

Ubuntu will not boot on this machine when discrete graphics is enabled in the BIOS, even in Recovery Mode. Changing the display settings to integrated graphics (or Optimus) allows the machine to boot, but then external monitors aren't recognized. This configuration also causes glxinfo to give the output 'Xlib: extension "GLX" missing on display ":0.0".' Installing the nvidia-current package hasn't fixed this problem.