Is your thread on a thread stand? That will probably work better if it's not already on one. You could try a smaller spool of thread and see if that solves the problem. That would eliminate the possibility of it being the tension assembly.
Next, I would take the tension assembly apart and clean well. There are free diagrams available if you don't have one. If you can't find it, post again and I'll look for where I got mine.
Good luck.