hmmm..
Have you tried un-installing the nvidia driver, and use the 'nv' driver instead of 'nvidia' in xorg.conf?
Also, if everything fails, and you have a good backup, try install the driver from nVidia. I have just updated the driver to 190.53 to be included in synaptic, but it's not included in synaptic yet,
Also try and knock your kernel version one notch either up or down to see if it's kernel related,
cheers,
MBantz