I suddenly had the same problem with my 301. I unthreaded, took the bobbin out and cleaned it, put the bobbin back in the casing rethreaded, still had a problem. Did it all again and got the flashlight so I made sure I got everything out of the bobbin area. Rethreaded again, put the bobbin in the casing again. Oh, and I put a new needle in the first time I did this whole thing. Everything was fine. I guess I just had a bit of lint or thread stuck in the bobbin area. You have to clean really well in that area. Just a tiny bit in the wrong place messes things up.