There is a good chance that a plastic gear is broken. They become brittle with age. This may be a case of having to replace all the plastic gears since they may break soon.

I'd look into fixing your old Singer. You'll be glad that you did. If that's not possible, then look for another oldie but goodie. The old ones are made to last several lifetimes.

I am a Bernina girl for life, but I have purchased two Brother machines from Costco for my daughters, and they have been happy. When you buy from Costco, you are getting their wonderful return policy.