If you wanted to look for some sort of systematic errors you can try this, Test Pattern Generator
The only thing left I can thing of is rotate all 6 of your tubes 20degrees. in case you have a rough spot or something the bearings are hitting. You can move the pen to a bad spot in the drawing and check each bearing position for a bad spot on the tube.
This sucks for me as I am sure I could find it in person in less than 15 minutes.