My 15-91 does this thing where, after letting up the gas, the needle still goes up and down a few times before stopping. Not really a big deal, but when quilting, it leaves little "french knots" of thread where I stopped the machine to adjust my quilt. I bought a new pedal and DH put it on for me. I thought maybe that would help but not really. Any ideas? I think I may need a new motor. Is that a difficult thing to do/find?