TFSR is one of the best sources for information on fixing old sewing machines.
http://www.tfsr.org/publications/tec...achine_manual/
You have something going on with your top tension most likely.
I wouldn't be surprised if it has a bit of gunk caught in it. That or there's a little pin that slides back and forth inside the threaded shaft that sometimes sticks. It could be that.
Don't be afraid to take it apart and clean it yourself. It's not difficult.
Rodney