I use Ubuntu 16.04. I have no Nvidia card, just Intel HD3000.
I can work for long time with just one user. When I try to change user 2-5 times all system freeze and I can do nothing. I can switch to console with ctrl+alt+FN only after some time (one minute or something like this). I tried to kill all X and lightdm processes and stop lightdm service and restart it - it doesn't work.
I can have working desktop only after reboot.
I attach syslog fragment where I tried to log in as guest, got system freeze, killed all X and lightdm processes, restarted lightdm with "systemctl restart lightdm" and tried to log in as usual user. It's common for all users, not just guest. Also system freeze happen even if I not just switch users but log out and log in. I also tried to move .config/dconf and it doesn't help.
May be bug somehow related to this https://bugs.launchpad.net/ubuntu/+source/ubuntu-drivers-common/+bug/1369216
I can restore to working state if I switch to GUI, close my laptop and it will sleep. After wake up system works.
Attach another syslog fragment