Tension set too high, dirty machine, crummy thread, burr on the thread path somewhere, burr on the needle plate hole, burr on the bobbin or bobbin case, burred or bent needle.

Clean your machine, rethread with a new needle right way around and try again. If it persists, stretch a piece of thin fabric like pantyhose nylon or thin lining fabric over your forefinger and feel along the thread path for burrs and dings. If minor, they can be gently polished out with crocus cloth.

Tension can be the problem but, a lot of the time tension changes becausr the machine needs a good cleaning and lube. Lint builds up in the mechanism as you sew from the needle punching it's way through the cloth. A machine needs to be cleaned after each 8 hours or so of sewing.

I would have to see it to be sure what's going on, but my best guess would be a tensioner is off. Follow the thread that is breaking back from the needle and look for a place where it goes through a little circular plate. It should have some sort of nut on that part. Give it a 1/4 turn counter-clockwise. If that doesn't fix it, turn it a little more.

It should look something like this, but every machine is different

http://www.april1930s.com/assets/images/...