This doesn't sound good. If you have double checked and you have threaded bobbin and needle correctly and you have checked for tension. ...and you have checked your manual ... Then you better go in for repair. I don't have your Janome but when my machines give me thread bunching problems, it's because I've threaded wrong....or I need to change to a new needle.

Now if it's stopping when you are giving it power, that's a whole 'nother thing. That needs repair. The two problems may be connected or not.

I hope you can resolve this soon. It's really frustrating to be without a sewing machine.