As Raggiemom said check the rethreading especially if you use cheaper thread. It comes unthreaded real easy. That might be why it works for awhile and then you have problems.
I would take it back to the dealer, but get Aurifil thread. I am sure she will be happy with that.
Gutterman seems to work better than Coats and Clark, but not as well as Aurifil.
I know how mad you get when it doesn't work. My service person has one and can use any kind of thread in hers. I just wish Janome would stand behind their product when there is a problem.