Finally here's MDK.
This has strange Zbuffer problems, or thats what it looks like.
It seems the the buffer isn't being cleared, and the screen fills with multiple images of the same object.
Ha, I was just going to say it doesn't sound like a zbuffer problem, but just as I was going to hit the first key, I realised you are probably right.
I was going to say that if the zbuffer was not being cleared, then progressively less and less of the image would be displayed, and so you would see more and more of the black where the frame buffers are cleared before rendering.
But what I just remembered is that lots of game don't clear the frame buffers because the sceen covers the whole screen. So the z buffer not being cleared would have exactly the effect you say.
I think Fabio recently cured a problem with zbuffer clearing, but I guess not this one.