I recently spent many hours solving a problem with my LR3 and wish to share my experience so that others may benefit.
I built the LR3 quite a while ago and it does not get a lot of use.
I brushed off the dust and to my surprise requesting a move from home in the Y direction of just 100mm caused the gantry to move an extra 6 on the Y1 side. What the hell! What caused this?
To cut a long story short, fine saw dust had entered through the rectangular hole used to fit the belt and had become jammed in the teeth of the pulley. Not all the teeth were clogged so the belt did not show signs of slipping. I cleaned the pulley and re-assembled and all was good with the world again. I suspect the dust caused the belt to jump a tooth every so often but I am not sure.
I don’t have a dust collection system installed, and I’m certain this has contributed to the problem.
As a temporary measure I put a small piece of tape over the hole.
I hope others find this post useful.