Dell Studio 1558 Laptop Screen Problem !!

Hello I purchased this Lap Top about 5-6 months ago and about a week ago I began to have problems with the screen. There are these dotted lines going across the screen horizontally and eventually the whole screen turns white unless I hold the screen in a certain position. It's gone to the point where I have to keep holding the screen at a certain position so it wont turn white or "freeze." It's weird, the screen itself freezes except for the monitor itself. I don't know what the problem is! If someone can find a solution that would be great! Please and thank you!!