Currently Being ModeratedOct 22, 2013 2:54 AM (in response to siubing_leung)
I suspect that you have posted in an incorrect forum since there are no 21" MBPs, the largest being 17".
You probably have an iMac. If that is that case. repost there.
Take a picture of the display when this phenomenon occurs and bring the Mac and picture to an Apple store or AASP. It sound like a possible GPU problem.