nVidia updated their validation routine for the video cards.
It greatly effects the DVI's output clock. My card went down from 400Mhz (DVI clock) to 155MHz DVI clock. Just try to look for "clock" in your Xorg.0.log. I bet it went down below 211.2 MHz (needed to drive your display).
This forum is littered with casualties (from this "bug fix"). Join the club.
I just hope nVidia is going to remove this "feature" soon.