After a lot of fuzz with tension, different feet, thread, being aware of speed, hand movements and what not, I am free motion stitching! It works even on my machine. I was a bit discouraged by the model 15 users a few times, since it has a few advantages and is a quilters favorite.
Did you sort out your speed issues? I know some of the new foot controllers are very good and you shall have very good feel and even acceleration with them. Since it's important to your type of sewing I would suggest trying a new one, and not give up until you have something that works. I have the old bakelite type, and that's the bad one in comparsion with the newer types. I have fairly good speed control with it.