This was also my problem....now that I have the right bobbin it works fine.
Originally Posted by MamaHen
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!