I see. That's definitely annoying. It could be two things. Either your video card or your monitor. You could troubleshoot it as follows:
1. Make sure the connections are good. (You have probably already done that repeatedly, but I had to say it).
2. Borrow a friends monitor and hook it up to your computer. If you see the same behavior then it is your video card. If the monitor never blacks out then it's your monitor. If you find that it is your monitor, do a double check and hook up your monitor to your friends computer and you'd know for sure.
Hope that helps.
