A somewhat common problem with any DLP, irrespective of model. The DMD (DLP chip) is beginning to fail. What you're seeing are a scattershot of the DMD's micromirrors becoming stuck, as opposed to being normally capable of moving freely throughout their range.
Various factors, including heat and electrical effects are responsible for this failure.
Call Mitsubishi and complain.
If they don't help you you can:
Replace the whole optical engine (fairly easy but somewhat expensive)
Replace just the DMD chip IN the optical engine. This is a bit more difficult, but is the least expensive option. You might also find a technician in your area who can do either of the above for you.