OK, then if the problem moves around, it's definitely not pixels of even dust in the optics, it's more serious. And the usual suspects are the capas scattered throughout this TV that always go bad. In your case, probably on the formatter board. This is one of many known problems on this TV.
But if I may, I'm going to give you some frank advice.
If you are at all inclined to junk this TV, in my opinion now's the time. This set was a dog from the get go.
Another problem just waiting to crop up is a defective DM module. The way that will happen is that one day, the front green LED will just rapidly flash, like it does when you plug the set into the power outlet, only it won't stop.
Mits has a program where they will rebuild the chassis of this TV for about $300, but you'd also have to get a Mits tech involved, and his time to pull the chassis and send it off, then reinstall it would be in the $200 range at the very minimum.
There are a couple of other bad design flaws in this TV, but the above is certainly enough in my book. Mits used to make great TV's, but ever since their entry into the DLP design (and again, this is my opinion), they haven't made a decent TV. Their current line is a step back in the right direction.
Sorry for the bad news, let me know if you have any other questions, thanks for the accept, and thanks for using JustAnswer!