It sounds like the problem is the BOBBIN not the machine. Incorrect size bobbins will hop around so use the correct size for the Janome, I use only bobbins made for Janome's. A lady in our guild was using off the rack bobbins in her Janome and couldn't figure out why she was having so much trouble, gave her a Janome bobbin and it solved every problem. Don't blame the machine, blame the parts & pieces!