I don't have that machine so I don't have any pointers for you but I can totally sympathize with you. My last quilt was that same one thing after another deal and I began to think it would never end (it actually hasn't ended because that quilt is at the cleaners trying to get a ball point ink stain out. Don't ask.)
I hope you can get the issue figured out and get on with your quilting! It definitely looks like a tension issue from the bobbin side of things. Looking forward to seeing your bargello all finished! (I know you're looking forward to that as well!)