I would suspect an electrical component is failing. Try your monitor on another computer and see if it does the same thing. If it does, the monitor is failing. Also/Or try another monitor on your computer. If the new monitor does the same as the old monitor, then maybe the video card in your computer is failing. Try a new video card. Keep trying different component combinations until you decide what has to be failing.
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I would suspect an electrical component is failing. Try your monitor on another computer and see if it does the same thing. If it does, the monitor is failing. Also/Or try another monitor on your computer. If the new monitor does the same as the old monitor, then maybe the video card in your computer is failing. Try a new video card. Keep trying different component combinations until you decide what has to be failing.