Yep, it's the coating on the banner material. Doesn't react well with the latex ink.
Don't want to assume anything but is it some vendor's economy vinyl?
Try Grimco's key banner, works great.
That is the pattern from you pinch rollers that happens on some material. Try turning you dry temps down a little. And, it usually shows up a the beginning of printing which leads me to think that the pinch rollers get heated a little more and some vinyl or coatings react this way.