Is there a hairline crack in your needle plate? Also check your thread path. After 50 years a guide may be worn thin and cause a problem, same for the bobbin area look for unusual wear and compacted lint in the bobbin case. There is a groove on the inside of the bobbin case - near the top - that over time gets filled with lint and dirt. remove the case from the machine and using some alcohol and a toothpick, clean out the groove. While you have the case out clean out any dust bunnies and put a drop of oil on the hook.