I have an old D620 that was overheating, I dismantled the laptop (have had prior experience doing this), got rid of a dust bunny the size of Rhode Island that was lodged in the fan, re-assembled the laptop and now when I boot up, the power light comes on, the numlock light is solid, the caplock and scroll lock are flashing (nothing on the video screen), it does this for maybe 30 seconds and then powers down. I've tried reseating the SODIMM a few times but it doesn't solve the problem. Any other ideas what could be causing this?