I'm not a Zenith guy but I'd be checking a few things first, make sure the cooling fans are running, make sure you replaced the bulb with the correct bulb, if you still have the old bulb, bad or not, try swapping it out again. New bulb could be having issue with weak ballast. Did you reset your bulb counter in the service menu?
Like I said, I'm not an expert on Zenith DLP's but it sounds like it's either getting hot and the DMD is kicking off or there is a power issue with the ballast. I'm replying to this because I've seen not body else seems to have anything to say. At least you can try a few things before junking the set.
2004 is pretty old and you can easily buy better, newer DLP's on craigslist for example pretty cheap. I just purchased a sony DLP that is the LCD projection type without color wheel for $200.