Try the following to fix:
1) At your desktop, right click your mouse and select Properties
2) When the Display Properties window comes up, click on the Settings tab at the top right.
3) Then click the Advanced button at the lower right.
4) Click on the Monitor tab at the top.
5) Check the Screen Refresh Rate....it should be around 75 Hz to avoid flicker.
If your refresh rate is lower (like 60 Hz) then try setting it to 75 or 80. However, just to be on the safe side, make sure your monitor can handle the higher refresh rate first!! Google it to look up your monitor model number and check its capabilities. Once you know that it will handle 80 Hz (most CRT monitors will anyway, mine goes to 100 Hz, but I'm just being extra cautious

) then go ahead with the above steps.
Btw...noticeable flicker is due to low refresh rates. The electron beam at the back of your monitor is constantly "painting" an image top to bottom on your screen at an extremely fast pace. However, 60 Hz is dang quick, but not quick enough since the human eye can sense the "refreshing" of the lines on the screen. When you raise the refresh rate, it's fast enough that our eyes can no longer sense any flicker. The screen looks more stable and is certainly easier on the eyes
