I bought my Bernina 830 4 years ago. For the first two years my thread kept breaking. I was so frustrated! My dealer is 2 1/2 hours away, so it's hard to go to him when I need to. Well, I finally took it to him with the thread on it that kept breaking and he chuckled and grabbed a 100 needle and put it on and NO MORE BREAKING!!! He told me the thread was too thick for a 70 needle. Fixed my problem right up. I haven't had any other problems with my 830 other than that and that was owner error completely. Now I use 50 wt. Prescencia thread on a cone with a 70 needle. It works just fine.
A size 100 needle, you must have been using some thick thread! That's the size needle suggested to use with the gold denim decorative thread. 50 wt 3 ply thread is too thick for a size 70 needle to give a balanced stitch on any of my machines.