I had the same problem.
I am not sure if you still have this problem or not, but I have an Epson GS 6000 about 6 months old, and I just ran into the same thing. What finally fixed it was "cleaning the encoder strip". This was after all kinds of testing, different media, different profiles, relinerization and everything. Finally the 5th Epson Tech Support guy that I was setting up a service call with, suggested that I try cleaning the encoder strip, and that fixed it!
Good luck, and I hope that this helps anyone else that is having that dreaded vertical banding problem.