It could be the bobbin, but I have what appear to be vintage bobbins (at least I didn't buy them) and have had that problem. The way I would describe it is that top thread repeatedly wraps around the bobbin case and somehow appears to get hung on something under the feed dogs. It locks up the machine with your fabric under the presser foot. You have to cut the thread to get your fabric out and by then you can't tell what it's hung up on. It happens in the middle of a seam, not at the beginning.
I was ready to switch machines until I did the cleaning as shown in
this video, which is the same one as referenced above. It's only happened once since then.
bkay
This is what I got out of the bobbin area after what I thought was already clean.
[ATTACH=CONFIG]604857[/ATTACH]