I have an 820 with the same issue. I took it to my dealer, and had a heavier spring put on it. The next quilt was beautiful, not a skipped stich in sight. I was so excited. The next quilt was a batik, and my "masterpiece" so far in my quilting career. It skipped a few stiches once in a while, but had no pattern to it, and I could go several passes without any skipping, so I started to think it was the fabric. The next quilt, I had a few skipped stiches at the beginning and then it was fine. This last quilt, that was a not a batik, I had a few, but did not see a pattern to it. I need to quilt a batik top and see if it gets worse. I use a 90 microtex needle. I clean and change needles after every quilt. I read the posts to this thread with great interest. I am curious to see what others with the same issue have discovered.

I read that Bernina did not test the new 830 enough before putting them on the marke and now is scrambling to get attention away from the 830 to the new 700 and 500 line. I have an older Bernina and it is a joy to use. I doubt any new Bernina will be as good quality as they use to be.

Quilting batik with an 820
 

Same problem as yours :
I tried several needles to quilt batik. I always had many skipped stitches. Our main problem (my friend and I) is that the "management" of Bernina France believes that we are not able to quilt correctly. We have better results with the lovely Aurora 440. Also I have an old 1001 which works better than the 820. We also have a problem with the needle threader. We experienced a lot of different threads, we tried the super titan needles, different batting, etc.. Who has some "tricks" to help us ?