It could either be a loose cable or your monitor or graphics card could be wearing out. Check the cable and if it's tight and secure at both ends, it will be either your monitor or graphics card. See if you can get another monitor to try on it and if the problem stops, then you know it's the monitor. If it's a CRT monitor it could be a sign that the tube is starting to wear out and if it's a TFT monitor it could be a problem with the backlight. You might be able to get them repaired, but it would probably be cheaper to buy a new one.