I had a problem with the pen falling over to the chassis when it went more than 30 degrees or so from the vertical. I thought the screw in the pen arm backer was loose, then I thought maybe the motor current needed adjustment, but nothing worked. Turns out the glue holding the shaft collar to the wood pen arm backer came loose. Not sure why, I wasn't storing it in the cold, or dropping it. I'm gluing it back in. Anyway, if your pen falls to the sides, check out the glue in the pen arm backer.

Hi Michael,
I'm sorry to hear about the trouble. Superglue is a good way to fix those. And, please feel free to contact our shop directly if you'd like a replacement pen arm backer: http://evilmadscience.com/contact