Prism1 thanks for posting that link. Though I've cleaned my encoder strip several times, I never thought to clean the slot in which it runs. So I've gleaned that information today thanks to you. I have used compressed air to blow out the area around the sensor's "eye" with good success, also.
Original poster, is your printer in a dusty environment? If so, try to limit that as best as you can. We are in the process of constructing a floor to ceiling room to house our saws, even though they are in a completely different room and one of them is equipped with a dust collector.
This we feel will further limit dust creeping into the design area.
Good luck.