Tension is the problem but a friend of mine just had her machine checked and it was the tension spring on the bobbin that had to be replaced.
Sounds like your top tension. Could be a piece of thread in the tension disk/guide. Start with a new bobbin and re thread after you have cleaned real good with a vacuum. Lots could be the problem. Usually tension is the bad guy. Is your spool in correctly with no pull on it? Is the bobbin in correctly and wound correctly? Never wind thread over another thread. The needle might need replacing. If all is correct you might need to get the drag/pull off the quilt by supporting it, the combo of hands moving and foot pedal is something that takes a fair amount of practice. Maybe you have all this right, just remember top tension is watched looking at the underneath of the quilt. I wish you well my friend.