I have a model 385.1264180 Kenmore sewing machine. It's about 25 years old now. Now when I press the foot pedal, sometimes it doesn't sew, and I have to slam the pedal to make it go. Sometimes it sews faster or slower than it's supposed to. Could this be that I only need to replace the foot pedal, or do you think it's a motor problem or an internal belt issue? Thanks.