Very interesting. It would be interesting to hear back from lancaster.d if the problem was resolved.
FWIW:
Several years ago, my MX850 behaved very similarly. Very, very frustrating. I just kept trying things ... randomly ... for days ... until it finally decided to work ok. Several times, I was close to giving up on a really, really nice/fast CLI-8 printer.
My speculation, in hindsight, is that I may have done something "close enough" to what is described by Adrian. Wish I'd seen his guidance earlier.
BTW ... send message to Lancaster.d to ask him to provide follow-up, especially if MP600 ok.