This could be a problem with the LCD or the video card. Try plugging in an external monitor to the laptop while the problem is occurring. If the problem carries over to the external monitor, it is probably a problem with the video card. If it does not carry over to the external monitor, then it is more likely a problem with the lcd screen itself. Where the laptop is only 3 months old, I would contact dell and have them fix the laptop. It should still be under warranty so I would let them fix it for you!
