The 99s have some weird features in the bobbin area that act opposite of what I would typically expect. easily 2 out of 3 99s I get need some pretty good adjustment in the hook area thanks to previous tinkerers.
Try this post from a couple of years ago:
http://www.quiltingboard.com/vintage...g-t195869.html
At the end of the day, if you want to sew and not tinker with the machines, I'd say stick with the ones that have treated you the best.
