Thank you for the reply.
The dot that you're seeing is not dust, and is actually an internal problem with the TV that requires a component to be replaced.
I will try to explain this the best that I can here.
This problem is one that is common with your set, and any DLP that uses a "DLP" in the Light Engine. The DLP chip which contains thousands of tiny mirrors on it is the culprit, and each spot that you see, is one of the tiny mirrors failing.
The problem will continue to get worse until the DLP chip is replaced, and there are no settings you can adjust, etc. that will fix the issue. Mitsubishi is aware of the problem and has stepped up to the plate to cover half of the repair. The chip itself is $250 and the cost of labor is also $250, with Mistubishi covering the cost of the part, you will be left with the labor charge.
You should contact Mitsubishi at the number below and tell them that you have a failing DLP chip and you would like them to fix the problem.
If you need further assistance, let me know and I will be glad to assist you.