I have worked with these computers before, and from experience I think it is the video card.
There was a problem with laptops from around that time, if they used Nvidia graphics chips. (NVS 110M for the D620/D630?)
If you have an Intel one, then It's most likely something else.
If you have the Nvidia thing, Google "GPU Reflow" and you'll find ways to do it.
It usually doesn't last very long.
If you want it to last long you should reball the GPU. Unfortunately there's no easy way to do it at home...yet.