I had a problem like that years ago on my Bernina. Could *not* figure out what was wrong, so finally put the project away. Years later the problem was gone. I think in my case there was a tiny thread caught somewhere in the upper tension or lower tension mechanism, and it eventually worked its way out.
My suggestion is to take the bobbin assembly out as far as possible and clean everything. Clean lint away from under the feed dogs too. You can also run some *unwaxed* dental floss around your upper tension discs in case there is a tiny piece of lint or thread caught in there. (Do this with the needle up.)