If you know how your projector works it would have save you the cost of the bulb.http://electronics.howstuffworks.com/projection-tv1.htm
Projectors are like the one use at the movie theater; If light is busted; it's black. If no flim is presented thers white light and if the film is scracth you see it on the screen.
If you don't see this problem other than watching a TV then it's your tunner.
If you see this problem even playing other video signals, then it's the image processing.
You need to take the TV to a shop if you really love your projection unit. Cost of replacing that parts could be anywhere between $200 - $500 depending on the component.
New 42" LCD is around $1000.