Like Jack said, you don't need pumps. Although the name sounds like they're pumping ink for printing, they're not.
After you're seeing the "ink starvation", stop the printer, bring the heads over to the service side, and look at the face of the heads. Are you seeing ink accumulation?
We had a very similar problem and could not for the life of us figure out what it was. After replacing a few parts, we realized it was not a mechanical failure at all, but static causing accumulation on the heads themselves. We added moisture to the air and the problem has been gone ever since. XJ-740 here too.