I don't know, because I can't see it, but it sounds like you may be slowing down without realizing it. That would be the thing I would think if I'm getting snarls of bobbin thread. Something is causing the thread to build up down there. Either that, or there could be lint in your bobbin case - which will affect tension. Try checking for lint first, then speeding up a little bit and see if that helps. Moving a large quilt feels very differently than moving a small test piece.
Hope this helps :)