This is almost certainly a hardware problem. The question is whether it is the video card or the monitor that is causing it.
When you say "blanks out" do you mean that it goes into standby mode, or just that the entire screen becomes black but the monitor stays on?
Is it a CRT monitor or LCD? Have you tried the monitor on a different machine, or a different monitor on the problem machine?
I have seen televisions shut down when they get too bright, due to electrical problems, so it's not inconceivable that your monitor may have some problem which causes the same thing for a different reason.
Try swapping monitors and computers, so that you can at least isolate the problem.
is a description of a similar problem relating to CRT televisions.