so, i've had my 15" powerbook for five months. within the last week, the screen started flickering whenever i adjust it, move the computer or sometimes just bang the desk it's on. also, the bottom half of the screen will randomly turn white, with a few horizontal lines through it, and will remain so until i give it a quick jolt. anyone have experience with this problem? is my screen about to crap-out? is my card loose? is something broken, or about to break? please, help. thanks.