I believe this same issue was causing me problems immediately after I built my Egg-Bot. Once I completed the build, I hooked it up to my computer, and tried to start drawing via a sample Inkscape file. the pen arm would just flop to the side, and the pen head would just jump up and down. I don’t know if I got more than one issue going on here. I will re attached the shaft collar to the wooden part, and hopefully this will fix the problem.