Ohhhhhh I feel your pain! I have a Janome and also deal with shredding thread and have tried Everything. A top stitch needle does seem to work best. One other hint that I don't think I've seen here is to place your top thread spool further from the machine (say in a coffee cup on the side of the machine) so that the thread has a longer distance to "relax" before it gets to the needle. (Maybe you tried the same thing via the cone holder.)
You've tried a new bobbin, right?
Since I haven't completely resolved this with my machine I am wondering if there is a burr someplace.