Acer LCD monitors have very narrow view angles, resulting in exactly the dark/light variation you describe if you move your head up and down even a small amount. It's important to set the angle of your monitor so it is 90 degrees to your line of sight. If you can't tilt the top back enough, try lifting the whole monitor on some books.

I have an old Acer which started to die a few years ago, when starting up it would just flash once and then go into sleep mode. I opened it up and found some swollen capacitors inside. $2 for some new ones and 10 mins with a soldering iron and it has been going strong ever since.