You MAY be sseing this bug in 12.04, filed specifically on a Dell 1330m though could apply to other Dell laptops with nvidia cards - https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers/+bug/940564
There were 3 possible solutions here on my 1330m with 8400m GS card -
Downgrade nvidia-current to the 290.XX version, packages for 12.04 available here - https://launchpad.net/ubuntu/+source/nvidia-graphics-drivers/290.10-0ubuntu2
To downgrade, download the .deb, then use sudo dpkg -i /path/to/nameof.deb
Switch to the nouveau driver which works ok but for many isn't as good. Can be improved a bit by enabling vsync in nouveau which is done in xorg.conf
At least here this only affected a 32 bit install. So dumping that install & starting over with a 64 bit install & shutdowns/restarts have been fine.
Otherwise wait for a new driver version where this doesn't occur, haven't seen one yet here