Looks like you have a Janome. I've had the same problem. One of the issues can be with the FMQ foot you're using. You got one (clear plastic) with the machine, and then you probably purchased the metal circle one shown in the picture. Switch to the clear plastic foot and see if the problem persists.
When you use the metal circle foot you have to adjust the height (sorry, I can't tell you whether to go up or down, so you'd have to experiment).
There might be a burr someplace. Try a new bobbin also (the actual bobbin might have gotten damaged).
I've been in the same situation as you...changed needle (topstitching works best), cleaned and oiled machine, changed bobbin case (red dot vs blue dot), changed needle plate (to single hole), and so on. And it happens when I'm about 75% of the way through a project so I don't want to change to a different thread. Arrrrrrgh!
SOMETIMES it can be due to a timing problem, but I think a dealer/repair shop has to fix that.