Bottom thread means prob on top. I own this machine, and I'm guessing it is not going thru the tensions correctly because maybe you have some fuzz/thread bits wrapped in the bronze bearing. Or it might be a burr on the blk plastic bobbin case. Take it out and run your finger around. I've learned how to do most of my repairs by following instructions in the manual posted on line..
http://www.annsultfixit.com/ultfixit.html And Ann will help you via email also, if she can.