Sounds very much like you have a failing monitor or power supply. If it's flat screen and low voltage from a power supply then borrow a power supply from a friend and see if that cures the problem. Check the voltages and power out put before you connect but they are all similar.
If it's a CRT Monitor, old fashioned large affair, then your monitor capacitors are failing. Check with freecycle if you have one in your area and you should be able to get one of these for free.