It looks like you are onto it. Go over the machine until it runs smootly, even with the bobbin winder down. Don't hesistate to oil and test sew a bit a few days in a row. I think this model has the later lubrication free motor, it has some type of brass or bronze bearings in stead; there might be variants on this. There is a roller under the feed dogs, it moves in a cam like part. This round bit should be turning like a wheel, it often needs extra attention.