Good suggestions from the previous posters, here's a couple more:
What media and profile are you using? There is a feed calibration built into most profiles that may need to be overridden if you are not printing the exact media that the profile was built for, or the profile could be putting down a little too much ink for the same reason. Those sorts of issues can vary by color and will tend to show up most in mid-tones.
Also, have you updated the VP-540 with the latest firmware that adds Roland Intelligent Pass Control? With that feature and an RIPC-enabled profile, banding will be reduced.
Feel free to PM me, or give Tech Support a call at (800) 542-2307 - Press '2'.