I really hate it when the problem is intermittent. It makes it a lot harder to find. Mine was doing that for a while and I had a tiny piece of thread in the take up lever mechanism inside the machine. How it was managing to interfere with the stitch, I don't know, but once we got that out of there, it stitched fine. Now, if I have a problem, the first thing we do is take the screw out of the back of the front cowling, take it off, and look at the entire stitch path.
Another time, I had managed to put an "L" bobbin in my Brother which takes a "15". The big Brother did not like it at all. It wouldn't sew good as long as yours did. Just a few stitches and it would throw a tantrum.
Right now, I am using a large cone of thread for some piecing. I am using a free standing thread holder, but am having to keep the carry handle for the machine in an upright position so that the thread runs over that before it goes through the guides or it tries to bind up. That will give me a similar problem as to what you are having. The thread stand needs to be taller, but it is so light that if I try to put it on anything it falls over.
Hope you find your problem quickly.