Why does my son's D620 display flicker?

Does the machine do the flicker right from start up or does it have to run for a while before it starts the lines and flickering. If it takes a while it could be a heat issue. this laptop has a dedicated chip for video and if it has cooling problems the card becomes unstable and eventually will quit all together.1 Try blowing out the fan backward with compressed air get it as clean as possible. 2 try a laptop cooling tray dont let the vents get blocked by setting it on a pillow or even on you lap. It needs the airflow to stay cool. If it quits altogether the only fix is to reflow the solder in the chip this is done by removing the motherboard and heating the affected area with a heat gun or in an oven. Hope this helps.