pics or it never happened. its a good thing I love talkin' JV3's so much, because unresolved threads like this kind of wear on the soul
without seeing what's going on, and dialing-down the heat being your solution, I have a feeling you're getting media tunneling/buckling during the print. If there is a wavy-ness of the vinyl as it is being printed, the ink doesn't lay down as it is supposed to, essentially dropping colors in a repeated "Bell-Curve", so to speak, along the highs&lows of the vinyl and resulting in a rainbow-effect of vertical banding.
that, and improper alignment will also cause/enhance this.
sidenote: encoder issues would result in isolated (one continuous spot) vertical lines/defects, media rippling is usually more uniform.
@hbb, the added note of misalignment was directed your way, as you stated that uni-directional also helped fix the problem. its possible that your bi-directional alignments may be slightly out of whack and enhanced the rainbow-effect caused by your media rippling.