Post#2 » Sat Nov 12, 2016 4:35 am
On the inside of your Dreamcast, there's an arced piece of plastic protruding from the lid. That "pusher" is supposed to hit a tiny lever when the lid is shut. Over the years, a Dreamcast's components may get slightly warped, causing the pusher to just miss the lever, or to fail to hold it in place.
And that's probably your problem, too. It stands to reason that by removing the screws, the shape of your case changes just enough to make the pusher hit the lever correctly again.
I had a similar situation, where my Dreamcast would only work if I placed an object on top of the lid. I fixed it by gluing a flat bit of plastic to the end of the pusher, thus making it large enough to be sure it always hits the lever and keeps it in place.
-edit-
There are other possible explanations, though. It might also be a loose or corroded contact. But the lever problem seems to be fairly common.