Just an idea, could it be the monitor that getting defective?
An LCD monitor usually has a back-light that can "shut-down", specially when is been on awhile.
You can check this with direct sunlight or a flaslight and see that there is a display, just no light to see it normally.
Of course, it could be the video card, or even the computer. When this happens, does the caps-lock light on the keyboard still work?