Here are some thoughts and questions:
It sounds like a lamp issue to me. As I understand it, a new lamp starts out "too bright" and takes somewhere around 50-200 hours to settle in to it's normal brightness. The brightness then degrades gradually (maybe even imperceptibly) over time. Finally, just before it is ready to go out for good, there is a significant drop in output (kind of like what you may be describing) and it gets very dim. How many hours are on yours?
Fishing for other potential issues here, but, is your Z3 ceiling mounted? I believe that high-gain screens reflect light back towards the projector, which means that ceiling mounting negates the gain properties, and could possibly even make your picture dimmer (although I don't suspect that is culprit here). Is it just as dim when you project on a wall?
There are about 1,000 hours on my Z3's first lamp and it's still looking great running in Eco mode using ROne's settings with the cc30r filter. Contrast and Brightness settings are both within +/-2, and Iris is around -30. It's paired with a 92" matte white Severtson pull-down screen (1.1 gain).
Most people think they're looking at a giant TV when they first see it and can't believe it's a projector. I'll have it running before company arrives, and when they come down the stairs and turn the corner, the typical response is, "Wow!", then, "How big of a screen is that?", then "That's a projector?", then finally they walk up to the side of the screen and look behind it to confirm that it really is just a pull-down screen.