Comment 7 for bug 1066410

Try modifying your /etc/init/lightdm.conf to the following (adding the sleep 2 line):

# LightDM - light Display Manager
#
# The display manager service manages the X servers running on the
# system, providing login and auto-login services
#
# based on gdm upstart script

description "LightDM Display Manager"
author "Robert Ancell "

start on ((filesystem
and runlevel [!06]
and started dbus
and (drm-device-added card0 PRIMARY_DEVICE_FOR_DISPLAY=1
or stopped udev-fallback-graphics))
or runlevel PREVLEVEL=S)

stop on runlevel [016]

emits login-session-start
emits desktop-session-start
emits desktop-shutdown

script
if [ -n "$UPSTART_EVENTS" ]
then
# Check kernel command-line for inhibitors, unless we are being called
# manually
for ARG in $(cat /proc/cmdline); do
if [ "$ARG" = "text" ]; then
plymouth quit || :
stop
exit 0
fi
done

[ ! -f /etc/X11/default-display-manager -o "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/bin/lightdm" -o "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/sbin/lightdm" ] || { stop; exit 0; }

if [ "$RUNLEVEL" = S -o "$RUNLEVEL" = 1 ]
then
# Single-user mode
plymouth quit || :
exit 0
fi
fi
sleep 2
exec lightdm
end script

post-stop script
if [ "$UPSTART_STOP_EVENTS" = runlevel ]; then
initctl emit desktop-shutdown
fi
end script