I have a Janome 6500 and I had this problem as well. I changed to topstitch needles and slowed down just a bit and all went smoothly after that. I use a LOT of Superior Threads and really like the King Tut. Good luck!
I was having the same problem also. Every time I change my bobbin thread I check the bobbin to make sure the tension on the bobbin is correct. I use good thread also, but when I started testing the tension on the bobbin and made adjustments, I haven't had any problems. There are also several different items on You Tube to show how to correctly test and adjust the bobbin. Good luck.
Georgetown CA I'd Rather Be Quilting