This is the most common problem that occurs with DLP projectors. The problem you're seeing are mirrors failing in the DLP chip. The Light Engine DLP chip has thousands of tiny mirrors- each one representing a Pixel. When the Pixels go white, it is a mirror failing. The problem will get worse and worse (this is what always happens), until your entire screen is white.
The cost to replace the DLP chip is around $400-$450 parts and labor, as the chip itself is close to $300 (Texas Instruments DLP chip, same used in most DLP TV's)This is not something I would recommend doing yourself as it is very time consuming to remove the light Engine, and it is best to have the set in the hands of a tech in case something goes wrong in the process.
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