If I'm not using the mouse or keyboard, my screen darkens about 20 percent after about 30 seconds, kind of like a pre-sleep. If I'm talking to somebody about what's on my screen, or just reading something, it's really annoying. I have to move the cursor for it to go back to normal. I have looked everywhere in prfereneces to try and change this, to no avail. Any help would be appreciated. thx