I had similar problems with a couple of my machines and with the first one it turned out to be a minor tension adjustment which was fixed by the nice little old man that lives next door to a coworker and has worked on sewing machines for the past gazillion years (he's 96 years old!)
With the second one, I had switched out bobbins, based on color of thread and turned out the two, although they looked alike were not interchangable.
Another time I was having fits with my machine and it turned out to be a burr on the needle.
Don't know if any of these would apply in your situation, but that's what happened to me.