The 301 did that to me recently. I was surprised since it's not a usual occurence with that machine. I tinkered a bit and it didn't get much better so I went back to basics. Unthreaded it, took the bobbin casing out, cleaned the bobbin area, put in a new needle, rethreaded, put the bobbin back in the casing and started again. Still not right. So took out the bobbin casing once more and got my flashlight. I cleaned once again and found some bits of lint in the bobbin casing area. Cleaned them out, put a drop of oil there and put it back together. Sews like a top again. It doesn't take much lint in the the right place to throw things off.