There have been a few maps (mostly in the AC server, but a few elsewhere) where there is a pretty horrible flickering - not the usual Z fighting issue where map designers badly place objects, but where the display appears not be rendered at all. Leave it long enough and the nVidia driver will crash and then restart
The odd thing is that ALT+TAB into Windows 7 and then back again fixes the problem - the only thing I can think of is that memory from the previous map wasn't totally cleared causing some corruption somewhere, and ALT+TABbing, as we all know, destroys all surfaces, forcing all graphics to be re-loaded.