Monitor

Is there still power to the monitor or is the screen just black? Do you hear any sort of sound when it goes black? Does the time the picture goes out vary or does it happen after say "X" minutes/hours everytime?

Do you have any power saving software activated that would cause the monitor blank out?
Check display properties?

Sounds weird but have you checked for any viruses or spyrus nasties?

If its hardware it could be many different things.

Let me know what happens.

Good Luck

Tim