Techno Jellies, what do you make of this? [details inside]?

I was given a Toshiba Satellite A45-S250, no, it isn’t brand new. Everything works great that I can see. However, when you 1st boot it up for about 1\250th of a second the screen projects bight and clear like you would expect. Then it dims out for about 3 seconds, then goes dark. If you act as if you will close the screen down and reopen it, the screen tried to light up again. I can use an external monitor, and it works fine. What do you make of it? I am leaning towards a hardware failure of some sort, but not being real familiar with laptops I wonder if it is a software thing?