Not sure what to tell you. You could try the new beta drivers from Nvidia. I'm using them, and they appear to run smoothly. However, installing Nvidia's drivers manually is pretty nasty, and I've never had much luck upgrading them.
If you want to try, get the proper driver from http://www.nvidia.com/Download/index.aspx?lang=en-us
Once downloaded, close out all your applications and press Crtl+Alt+F1. Login.
install some necessary files
stop the x server
sudo apt-get install build-essential kernel-source
replace kdm with gdm if you are using gnome
sudo /etc/init.d/kdm stop
install the driver
once you type NVIDIA you can press tab to complete the line. Follow the directions and restart when you are done.
sudo sh ./Desktop/NVIDIA...