Re: Please help with computer monitor
It could be that your video card is faulty or overheating. If this is a desktop computer, do you have a dedicated graphics card? Or, are you using on board graphics built into the motherboard.
If it's the former then check that the heatsink is clean, and that the fan (if it has one) is also clean and working. If it has multiple outputs (ie: DVI, and VGA) then try another output and see if the problem goes away.