Screen blacks out randomly

I'm having a problem that shortly after my computer boots up the screen will turn black. It happens at random intervals but the longest it has lasted is an hour. When the screen turns black I can still make out some of what's on the desktop but it is too dark to see even the mouse move.

I've ruled out monitor problems.

I took it to the Geek Squad and they found nothing wrong with it other than quite a bit of dust inside. They wanted to charge me $200 to clean it. I took it home and cleaned it myself.

I've tried everything I could find in online tips and nothing has worked.

Any ideas?