I am having the same problem. When moving the head around by hand (with UM off of course,) mine has always had inconsistent resistance in the Y direction, while X is smooth. The resistance seems to be enough to stop the motor for a few steps. Today it occurred to me that this could be due to several of my Y-axis pulleys being out of round (they sure look it anyway,) including the motor pulley. The heavy resistance is not reliably cyclical, so maybe it's when 2 or more pulleys are "in sync." That's all I've come up with so far anyway.