OK, good work. I'm concerned that it's something simple that has been overlooked. At this point, I'm out of "quick fixes". The next step would be to look at the ink delivery system, the ink delivery tubes, and a system purge, and replacement of all printheads and inks using setup carts/heads. This would be expensive.
Since the printer is not throwing any errors, it is difficult to impossible to discern whether it's the ink delivery system including the carriage, or any number of boards, of which there could be a bad connection, or faulty component causing the problem.
I have to say that from here out it would be shooting in the dark without being able to have hands on the printer. I'm hesitant to make any more recommendations without being able to physically examine the printer. Replacing one part after another is pretty much guess work and would get expensive.
I think that buying new inks, new printheads, etc., then new replacement parts, tearing the machine down to the frame, doesn't make sense without guidance or being done by a professional at this point.
I can't help but think it's something simple we've overlooked, but that's not always the case.
You can get a brand new Z3200ps 44" from ProImaging delivered for $3,000. If you feel you have nothing to loose, you could continue to tear your machine down, replace the carriage and a new belt while you are at it, and change out the trailing cable, etc., etc. By the time you buy all new printheads, carts, and components, I think you'd be better off just getting a new printer now that this one is as old as it is.
I wish you luck with it. Maybe someone else will have a fresh approach and find the answer.
Mark