I turn on my computer, and the screen is bright (normal, because of the way i set it). After i start working on it for a while, it turn a bit darker, making me think that there might be an inadiquate power source. I have no clue why it is doing this, it seems to turn darker when i am doing something that requres the processor to work. As soon as I am finished doing that certain task, the screen turns bright again. Any ideas why this is happening? Thx.