Yeah, I've seen pretty similar patterns on GS6000's before.
I have one client who has two identical machines who got pretty similar patterns in pretty similar colors on certain types of canvas. Breathing Color is their favorite brand, so these guys aren't scrimping. We did some pretty extensive tests involving trying different RIP's, trying different materials, and trying different lot numbers, and the bottom line is that in certain situations, the only way to get rid of the issue was just to go up to 1440x1440.
I wound up making them 1440x1440 profiles and basically, if they have dark bluish blacks, they just run them that way.
If you're running 720x720, and you haven't tried bumping up the resolution, that just might be your issue.
(If that's your issue, what I think is happening is that at 1440x1440, the GS6000 uses only its smallest dot size, rather than being a multi-dot printer. My guess is that in certain combinations, with the max-size dot, the media is rejecting the ink. So it isn't so much the resolution, but the dot size. Unfortunately, if you turn the mid and large dots off in 720x720, you just don't get enough coverage to get rich color.)