Hey I hope I can help. I was having very similar issues with my HD20 and found your post while trying to correct my own. My issues began when I replaced a broken color wheel. I wonder if the same did not happen to the previous owner. My solution was simple, however completely undocumented.
-Open up your projectors service menu
quick google shows you need to "hit the three buttons at the same time???
-Open up an image or video with extreme color banding
-Find the "Color Wheel Index" option
-Adjust this setting up and down until you find an area in which the banding disappears
I really hope this works for you and anyone else who is google'ing for a solution.