I see strange lockups on a fairly new install of Ubuntu 12.04.01 LTS, possibly due to the video card. I have an Nvidia GeForce FX 5500 in an HP ProLiant ML350 G6. With lightdm, if I click around on the login box to select the type of graphics session (Unity, Gnome classic, etc.), the machine "locks up", defined as the screen going black and getting no signal (monitor immediately goes to sleep), then the machine gradually bogging down (I can log in via ssh but things get slower and slower until I have to reboot). I tried switching to gdm as the display manager, and I get the same sort of crash but not in such a reproducible way.
I have nvidia-173 and nvidia-173-updates installed. I can provide log file info if someone tells me which file to look into.
Thanks for any help.