I have never seen a projector do what yours is doing. But it is either an optical part inside that is out of alignment or something that has shifted and is protruding into the image/light path. The only way to be sure and to correct is to open the projector.
If this was a NEW projector, I would say send it back. But since they stopped making that model in April 2009, you bought it used. Now, if the seller gave you some guarantee, then take it up with him and see if you can resolve this.
But there is no way this is normal for ANY projector.