- 1) The rubber wheels that transmit the rotation of the scroll wheel to the optical shutter wheel were not sitting straight on their axle. Easily fixed by pushing them into their correct positions.
2) The optical shutter wheel itself had a large sprue mark that was rubbing against some internal parts in certain positions of the rotation. This actually caused the scroll wheel's stiffness. Fixed with a carpet knife.
It seems that I'm not the only one with scroll wheel problems: http://geekhack.org/showthread.php?2835 ... roll-wheel