Great! I'm glad your problem is solved. I didn't mean to minimize your solution, as my problem might not have been the same. The thread would get tangled up in the bobbin area. I would have to cut the thread to get the fabric out and by then, you couldn't tell where it was hanging up. Similar to your situation, it was intermittent.
I had already cleaned the bobbin area in what I thought was extensively, but it was suggested that I do it again. I watched a you tube video on cleaning the bobbin area extensively with q-tips and sewing machine oil. I followed the example and the problem stopped.
I posted photos of the gunk I got out of the area, but couldn't find the post.
Barbara