I just went through that with my jv5. Its a bad set of ink I am almost sure. I was hoping to get others to let me know about this same problem as I think they need to adress this issue. I have been getting the old "we havent had this problem with anyone else" story.
I finally figured it out and came up with a way to back flush the head to get it working. I think this is a work around and not the proper fix, but it gets it working.
I hate to hear you are having this problem, I spent nearly 2 weeks trying to get it figured out, and a ton of money on parts and red label shipping.
Darren