PlontinusVeritas's suggestion to reset SMC seemed have solved my problem.
I have Macbook Air mid-2011 with Lion 10-7-5 OS-X.
I started having screen issue when I statred using an external display.
It seems happening randomly, but after reading this forum, I figure it was happening after the use of external display and/or when I put it into sleep mode. When issue comes up, I was able to temporarily solve the problem by adjusting the opening angle of the lid. So, I thought that it might be caused by an unsatble connection of the screen to the mainbody. The problem got worse and worse, I have been keep using the external drive.
So, I tried to figure out a solution and did google search to find this forum. I tried SMC reset and tried to put my Mac air to sleep, to turn off and start, and to restart.. and I haven't get the problem back yet.
I left my external monitor at work so I will have to try using it again tomorrow and to see if SMC reset really solved the issue.
If the problem gets back and then I can solve it by resetting SMC, I will be still very happy and thankful to PlontinusVeritas!