I clean the surface with Alcohol, and then use 400 grit sandpaper to clean the surface. I scrub the contacts to a shine and clean the surface again with Alcohol. You'll have to manually move the coil unit so you can get to all the rivets, or better yet, remove the finger disc.

From Clay's guide: "On a Williams stepper, there is a 7/16" nut which holds the finger disc in place. To remove this nut, on the opposite side of the stepper, put a small screw driver into one of the large holes in the stepper cog to prevent it from turning."

Once off, I carefully shin the bottom "fingers" of the disc with 400 grit to make sure the contact is solid.

A lot of people apply a thin (very thin) layer of Super Lube to help the steppers move easily.

http://www.amazon.com/Super-Lube-21030-Synthetic-Grease/dp/B000XBH9HI/ref=sr_1_1

Follow Clay's guide (2D):
http://www.pinrepair.com/em/index2.htm#top

Did you make sure all the wiper blades are making contact with stepper unit? You could also check that there isn't some "play" in the unit and the blades can be pulled away from the surface when it rotates.
You could also try cleaning the blades' contacts with some sandpaper.