It may appear that we're questioning your competence, but you have to remember we can't "see" what you can, in order to help all we have is the data you provide. I've converted a similar printer (Epson, cartridges ride with head) to an outboard ink system, with long plastic tubes. It worked ok for a little while, but it was not that reliable, I finally gave up. I did find it necessary to create custom printer profiles, since the inks are significantly different than Epson's. I did not have the type of problem you're experiencing.
Telling us "resolution and DPI are fine" does not give us much to go on. Many times issues on these forums are resolved because the original poster did go back over all the details he thought were correct, and found a mysteriously changed setting or whatever.
My best guess is plugged nozzle(s). My next request was going to be for you to print out a nozzle check, scan it, and post the results.
Good luck,
Richard Southworth