Does thw picture just disappear without the computer freezing up or restarting? If so, I'd definitely say a video problem. Check your cable first, of course, but it's likely the last driver update as other threads have discussed- some people had luck by turning off aero in vista, but I'd say that if the problem is not already fixed by driver, then it will be soon. See the thread below too.
BLACK SCREEN FIX - NVIDIA Forums